As stated above, this guide lets you run GW on the latest firmware via a rigged up EmuNand.
https://gbatemp.net/threads/gateway-gw3d-11-10-b9s-and-cfw.550040/
Poking through the latest Gateway release I found from digging around the internet, it looks like it may come with a .cia to be installed that's vulnerable to the MSET exploit (which you would be going for if you wanted the DS profile exploit more than a modern setup). I cannot, for the life of me, find a way to launch launcher.dat aside from the .3dsx method. There are numerous exploits for 9.2.0-20U, but if you wish for a recommendation, I would recommend Soundhax. If I'm not mistaken, you can use the GW launcher to reinstall MSET?
Truly I'm clueless. But I gave it my strongest suggestion.
What you are trying is possible; but you'd be hard pressed to find a soul that can recommend it in good conscious.
On the note of downgrading. Don't. The only remotely reasonable reason to downgrade a N3DS from 9.2.0-20U is to get your OTP from that old 2.1 version that was rigged up to get otp.bin back when Arm9LoaderHax was a big deal. B9S is the big deal now though. And the N3DS did not officially release with any version older than 9.0.0-20U.
I also wish to acknowledge the following.
https://gbatemp.net/threads/importing-gateway-saves-to-cia-games.425743/ - This is a resource for converting your Gateway save files
https://github.com/ihaveamac/custom-install - This is a downright divine tool for rapidly installing .cia files.
My recommendation is Luma. I wish you the best of luck!