I still use my Gateway, mostly because I don't use my 3DS a lot anymore and it's a hassle to get all my games again in CIA format and convert all the saves.
That being said, if you want to switch it's fairly easy, just a bit time consuming. You can use this tool to help you export your Gateway saves to decrypted format: https://gbatemp.net/threads/gateway-save-export-helper.465503/
It's a lot easier than the alternative, which requires manually renaming the .sav on the SD and exporting one at a time.
It's best to do this first, since accidentally updating (or not knowing not to remove your emuNAND when the guide tells you to do so) will leave you unable to boot into GW mode until you downgrade your sysNAND, or make an emuNAND and downgrade that.
Once that's done you can just follow http://3ds.guide assuming you don't have Gateway Fast Boot. If you have Gateway Fast Boot, the easiest way to get rid of that is to restore a NAND backup from before you installed it. I think there's a way to do it even if you don't have an earlier NAND backup, but I don't know the details.
If you would like to dual boot GW and Luma, keep your emuNAND and continue following the guide as normal, ignoring the "Move emuNAND" part, and make sure to never update your emuNAND past 11.2 or boot Gateway sysNAND mode on 11.3+ (the latter will wipe B9S from your system)
Once you're done with the guide you can follow this to regain GW support: https://gbatemp.net/threads/tutorial-using-luma3ds-gateway-on-b9s.431691/
This is the setup I have, although I used BootCtr9 as a bootloader so I can boot GW by default, and I hardly ever use Luma.
I MUCH prefer the method I outlined for the EmuNAND before. Since he already has A9LH(or B9S for that matter) Use Godmode9 to BACKUP EMUNAND. Then do the guide. when you get to moving the emunand STOP, BACK UP YOUR WHOLE SD. Then move the EmuNAND to SysNAND. When you are done, format the SD in godmode and create a new EmuNAND. Then use godmode to restore your old EmuNAND backup. Then Finish the 3ds guide. Sounds complicated but it beats making a new EmuNAND for GW. You can delete all the installed CIAs off the EmuNAND after that. I do that in Luma. This way you'll wind up with 11.2 EmuNAND already complete with everything you want. At that point, if you want online in GW, I would update all but 3 title to 11.6. I like a tutorial earlier in the thread.
I'm in a similar situation to the OP, with a 4.5 3DS, gateway and a CFW which hosts the majority of my games via CIA.
I've been thinking about upgrading the whole thing but one point that holds me back is, what do I actually stand to gain? I can run every game via CIA (not tried a new rom on the gateway for a year or so though), so for the sake of myself, and maybe the OP, what are the pro's and cons?
If I lost the ability to use the gateway (which I mainly use to store games larger than 1GB on a 32GB card), I'd probably have to upgrade the SD in my 3DS, which is another set of headaches but the nerd in me wants to tinker with new hacks.
Do both! Get Luma. Nobody ever was sorry for going to CFW. It's FASTER. Everything from boot to loading is WAY faster than old EmuNANDS. It's better compatility and Luma is safe. GW is fun and gives you the option of a second SD but Luma is way better.
If you want to keep using GW, do the dual boot route (luma on sysnand, and GW on emunand)
You can install b9s on your 3DS and restore all games from your emunand to sysnand.
It's all up to you.
Is gw that important to you?
The cheat engine function is already available on NTR AR.
See, your post shifts from helping to "Is gw that important to you?" Please stop it.
Not a whole lot besides some neat Luma3DS features (a lot of which you can get with NTR-CFW anyway) and support for the latest firmware. And never having to worry about firmware updates again as they're mostly painless with Luma.
It's mostly the little things. You certainly won't miss Gateway once all is said and done, but it is a bit of a hassle to switch.
FreeShop is much more convenient than .3DS files though. You can use FreeShop on GW too, but .3DS roms are like the one benefit GW still has, and it's not much of a benefit anymore.
Don't forget an extra SD worth of space for 3DS games. I think it's better to run both. If the task it too daunting. I might write up a good tutorial to do this right once and for all.