The gateway 3ds supports the version 9.2 and below. You can use sysupdater and download the correct version files, make sure you have the correct region, or you will brick your console, running it will update you to 9.2.
From 9.2 you can take your sd and put the gateway launcher from their website on the root of the sd. Then you need a way to run gateway.
You can install homebrew, and then menuhax, which should have been included with the homebrew starterpack. After you've set up menuhax, you can take the gateway.3dsx file that was also included with the gateway launcher, place it on the sd. Rename the boot.3dsx to homebrew.3dsx, then rename gateway.3dsx to boot.3dsx. Now your console will power on and run gateway. From there you can run the format emunand command, just make sure to back up your sd first. Then you can replace all the files that were backed up on the sd card again, and your 3ds should power on into emunand. Go into your system settings and check that gw3ds is next to the firmware version, if it's not then you're not in emunand, if it is, then you can run system update from the settings menu.
Now you're on the latest emunand firmware, and you can install Bigbluemenu or any title manager of your preference, just look for a guide on that.