SYSNAND-ONLY USERS: DO NOT BOOT GW MODE ON 11.3+ - YOU WILL LOSE B9S AND BE LEFT ON A STOCK FIRMWARE
I highly advise against the use of GW on SysNAND-Only Setups.
GW currently DOES NOT support firmwares 11.3//11.4 - B9S does not change that!
If you're on 11.3+ - Don't even try to use GW. Use Luma3DS or any other CFW that has confirmed support for 11.3+ firmwares.

Setting up GW with Luma3DS' chainloader

Requirements: SysNAND or EmuNAND on 11.2 or lower!
Tested and working on B9S 1.2 (Luma3DS 8.0)

1. Download the v2gw.zip attached to this post.
2. Extract the "arm9loaderhax.bin" from v2gw.zip to your /luma/payloads/ folder.
3. Rename that very "arm9loaderhax.bin" you just extracted to "v2gw.bin" or to whatever you want. However, step 12 will assume v2gw.bin was used, you can edit that reference to meet whatever you decided to name it.
4. Download GW's ‘arm9loaderhax.bin.zip‘ file.
   • On their main home page. Under the post titled: "GATEWAY 4.0B PRIVATE BETA #2"
   • Or on their openly public Private Beta Program page, on this page it'll be under the "» arm9loaderhax for users with existing a9lh installs" section.
5. Extract GW's "arm9loaderhax.bin" file from their zip file to your /luma/payloads/ folder.
6. Rename that very "arm9loaderhax.bin" you just extracted to "gateway.bin"
7. Make sure you have the latest GW's Launcher.dat on the root of your SD Card.
8. This dev-build of BootCTR9: https://gbatemp.net/threads/release-the-bootctr9-release-thread.435347/page-3#post-7370533
9. Extract the "boot.firm" file from the BootCTR9-firmv2.zip to your /luma/payloads/ folder.
10. Rename that very "boot.firm" you just extracted to "down_BootCTR9.firm" or to whatever you want that is allowed by Luma3DS' payload chainloader.
    - Other button options: https://github.com/AuroraWright/Luma3DS/wiki/Other-features-and-notes#payload-chainloading
11. Extract the "boot_config.ini" file from from the BootCTR9-firmv2.zip to the root of your SD Card.
12. Open the "boot_config.ini" file and overwrite everything with the following:
    
    Code:

    [BOOTCTR9]
    key_delay = 1000
    
    [DEFAULT]
    path = /luma/payloads/v2gw.bin
    

13. You are now done. If you followed this tutorial, holding DPAD-DOWN at boot time, will load up Gateway Mode.
    - SysNAND-Only Users on 11.2 will be prompted to press 'L+R+SELECT' to boot SYSNAND. DO NOT PRESS THE BUTTON PROMPT ON 11.3+
    - GW EmuNAND users will automatically boot into their GW EmuNAND.

FAQ

Q: How do I boot into the GW Menu?
A: Hold L+Select, as soon as you see the Dragon Logo.

Q: Can I use my GW Blue Card?
A: Yes, but only while using Luma3DS. It will not work in GW Mode.

Q: Can I play online while in GW Mode?
A: Yes, sort-of. But you have to either:

• Use CTR-HTTPWN: https://github.com/yellows8/ctr-httpwn/releases
• or use this guide

Q: I'm getting a black screen!? HELP!?
A: Multiple reasons for that below:

• Modified SecureInfo_A - GW does not patch this. You'll need an untouched donor file of this.
• Using a RedNAND - GW only supports GW EmuNAND, it cannot boot RedNAND. Use the GW Menu or GodMode9 to make one.
• On SysNAND-Only Setups - Not having properly removed EmuNAND//RedNAND. Use GodMode to properly format your SD Card. But I advise you set one up.
• Trying to boot an 11.3//11.4 EmuNAND - GW does not support anything higher than 11.2
• If it's not listed, then who knows. Do you have a modified Home Menu or font? Those might not work at all in GW Mode.

Credits

• @Hayleia for making v2gw.bin~!
• @RednaxelaNnamtra for BootCTR9~!
• @nedron92 for discovering this method~!

Many thanks to the above lovely people~ <3 ♥
Original posts:
Link: https://gbatemp.net/threads/using-gateway-on-a9lh-v2.431222/page-4#post-6464898
Link: http://gbatemp.net/threads/tutorial-using-luma3ds-with-gateway-on-v2-a9lh.431691/page-4#post-6472283
Link: https://gbatemp.net/threads/tutorial-booting-gw-with-b9s-and-some-help.471417/

Old Guide

I highly advise against the use of GW for SysNAND-Only setups.

11.3&11.4 Break GW. GW currently DOES NOT support 11.3 or 11.4. You will either brick or lose A9LH. More than likely it seems you will just lose A9LH.

Again, if you attempt to boot GW Mode, on a SYSNAND-Only Setup while on 11.3 or 11.4, you will lose A9LH. It does not matter if you safely updated to this firmware via another CFW that safely protects FIRM.

If you want to use 11.3+, you will not be able to use GW. Don't even try. Only use Luma3DS or other CFW that support 11.3+

If you are a Gateway3DS user, and have recently finished Plailect's A9LH Guide, then you should be on v2 A9LH. Unfortunately, at the time this thread was made, GW's a9lh.bin file is incompatible with Luma3DS' payload chainloading feature while on v2 A9LH.

So one way around this was to either forego using Luma3DS' payload chainloading, and use a boot manager that does work in initializing GW's a9lh.bin correctly. Like BootCTR9. Either you let BootCTR9 handle all payloads, or you set it up to chainload into Luma3DS and from there let Luma3DS do all the work.

Either way, some people might not like BootCTR9 (Big fan of BootCTR9, seriously thing is amazing~!)

So if you don't want to use the BootCTR9 method, then @Hayleia made a very stripped down boot manager that only loads /luma/payloads/gateway.bin. You still need GW's a9lh.bin in addition to this stripped down bootmanager, but that's all you need, these two files! No need for messy configs if you just want to stick with Luma3DS' payload chainloader.

Now on to the damn tutorials already~!​

~Choose your path~
Gateway Logo (Recommend for Everyone - SysNAND Only & GW EmuNAND Users)
No Gateway Logo (Not Recommend, especially NOT for SysNAND Only Users - Only for those allergic to the dragon logo - USE AT YOUR OWN RISK)
!~Important Note (DO NOTE UPDATE SYSNAND while in GW MODE)(PLEASE READ)
!~Getting Black Screen After Dragon Logo and After Following This Guide~! (PLEASE READ)
!~I want to use GW as Default~!
!~♥ Credits ♥~!
​

Gateway Logo

1. Download the v2gw.zip attached to this post.
2. Extract the "arm9loaderhax.bin" from v2gw.zip to your /luma/payloads/ folder.
3. Rename that very "arm9loaderhax.bin" you just extracted to "down_v2gw.bin" or to whatever you want that is allowed by Luma3DS' payload chainloader.
Other button options: https://github.com/AuroraWright/Luma3DS/wiki/Other-features-and-notes#payload-chainloading
4. Download GW's ‘arm9loaderhax.bin.zip‘ file.
You have to get this directly from Gateway's site. Which is either:
A) On their main home page.
B) Or on their openly public Private Beta Program page, on this page it'll be under the "» arm9loaderhax for users with existing a9lh installs" section.
5. Extract GW's "arm9loaderhax.bin" file from their zip file to your /luma/payloads/ folder.
6. Rename that very "arm9loaderhax.bin" you just extracted to "gateway.bin"
7. Make sure you have the latest GW's Launcher.dat on the root of your SD Card.
8. You are now done. If you followed this tutorial, holding DPAD-DOWN at boot time, will load up Gateway Mode.
However, if you are a SysNAND Only User, and on 11.1 and using the 4.2 Launcher.dat, you will be prompted to press 'L+R+SELECT' to boot SYSNAND.
GW EmuNAND users will automatically boot into their GW EmuNAND.

Notes: To Boot into the GW Menu: Hold L+Select, as soon as you see the Dragon Logo.
To Force a Manual Gateway3DS RedCard Update: Hold R+Select as soon as you see the Dragon Logo.



No Gateway Logo (Not Recommend - Only for those allergic to the dragon logo - USE AT YOUR OWN RISK)

Notes: This version will de-init the screen by default, and as such you won't see the Gateway Logo. Don't worry it'll init once you land on the Home. Due to how this works, only GW EmuNAND users should consider using this. This is because SysNAND users will land on the SysNAND prompt nag.

1. Download the v2gw-de-init.zip attached to this post.
2. Extract the "v2gw.bin" from v2gw-de-init.zip to your /luma/payloads/ folder.
3. Rename that very "v2gw2.bin" you just extracted to "down_v2gw.bin" or to whatever you want that is allowed by Luma3DS' payload chainloader.
Other button options: https://github.com/AuroraWright/Luma3DS/wiki/Other-features-and-notes#payload-chainloading
4. Download GW's ‘arm9loaderhax.bin.zip‘ file.
You have to get this directly from Gateway's site. Which is either:
A) On their main home page.
B) Or on their openly public Private Beta Program page, on this page it'll be under the "» arm9loaderhax for users with existing a9lh installs" section.
5. Extract GW's "arm9loaderhax.bin" file from their zip file to your /luma/payloads/ folder.
6. Rename that very "arm9loaderhax.bin" you just extracted to "gateway.bin"
7. Make sure you have the latest GW's Launcher.dat on the root of your SD Card.
8. You are now done. If you followed this tutorial, holding DPAD-DOWN at boot time, will load up Gateway Mode without the Gateway Logo.

Tested on N3DS and O3DS by me, 2DS by @Hayleia.
Should work on either screen-init or non-screen-init v2 A9LH.



Important Note: Never update your SysNAND in Gateway Mode. It does not matter what Launcher.dat version you use, if you go ahead and update while in SysNAND Mode, you will either brick or lose A9LH+CFW and be on the latest stock firmware. This is because GW Mode does not offer FIRM protection in A9LH. Updating your GW EmuNAND is fine and safe, and is what Gateway actually recommends you do and use even when using A9LH.



!~Getting Black Screen After Dragon Logo and After Following This Guide~! (PLEASE READ)
Sounds like someone hasn't properly removed their EmuNAND//RedNAND with EmuNAND9. The Black Screen problem is common to people who skipped doing it properly and used a third-party tool like EaseUS or any other partition manager.

Since EmuNAND//RedNAND wasn't removed properly, traces were leftover and GW by default sees it, and tries to boot into this non-existent EmuNAND//RedNAND and fails. So how do we fix it? We use EmuNAND9 to preperly remove traces of it.

Part 5, Section V of the old A9LH guide covers this. However, I will list all relevant steps here.
Link: https://github.com/Plailect/Guide_R...ax)#section-v---removing-rednand-from-your-sd

The new guide also sort of mentions if in this new page, and does show you how to setup EmuNAND9:
Link: https://plailect.github.io/Guide/move-emunand

Files we'll need:
Latest release of EmuNAND9: https://github.com/d0k3/EmuNAND9/releases/latest

1. Copy EmuNAND9.bin from the EmuNAND9.zip to the /luma/payloads/ folder on your SD card and rename EmuNAND9.bin to y_EmuNAND9.bin
2. Backup every file on your SD card to a folder on your computer, all files will be deleted in the following steps
3. Reinsert your SD card into your 3DS
4. Hold (Y) at boot time to open EmuNAND9
5. Go to "SD Format Options...", then select the "Format SD (no EmuNAND)" option, and wait for it to finish
6. Press (B) to return to the menu, then press Select to safely eject your SD Card
7. Put your SD card back into your computer, and copy all the files you backed up previously, back into your SD Card
8. Reinsert your SD card into your 3DS, then press (Start) to reboot



Additional Note: You will still have to look into BootCTR9 if for some reason you want GW to be the default, as opposed to Luma3DS. This tutorial is meant more for the people who want Luma3DS as their main//default, and want to use Luma3DS' payload chainloader to boot into GW Mode.
Go to the following link for the tutorial on setting up GW as your main with BootCTR9.
Link: https://gbatemp.net/threads/using-gateway-on-a9lh-v2.431222/


All credits to @Hayleia for making both of these~! Thank you~ <3 ♥
Original posts:
Link: https://gbatemp.net/threads/using-gateway-on-a9lh-v2.431222/page-4#post-6464898
Link: http://gbatemp.net/threads/tutorial-using-luma3ds-with-gateway-on-v2-a9lh.431691/page-4#post-6472283