Jakibaki has released an updated version of his GBA Emulator for Nintendo Switch:

I ported the gameboy advance emulator gdkGBA (originally made by gdkchan) to the nintendo switch! (Works on all versions as far as I know)

Unfortunately this emulator doesn't support in-game-saving (it crashes when you attempt to save!) so I added an savestate feature:
Press Plus+R so save the current state and Plus+L to load it again.

Simply put the rom into sd:/switch/roms/ and start the homebrew.

As soon as retroarch gets updated for 5.0 this will instantly become obsolete!

source (you need the sdl2-libraries from the devkit-pro pacman to build this)

Known issues:
Sound is a bit weird (it seems like sometimes the wrong instruments play)
After loading a savestate the sound gets even more weird for a while (or until the music changes)
Some games seem to crash the switch after a while.

Sound now much better now! Many thanks to @renhei for the fix!
Changed rompath from sd:/switch/roms to sd:/switch/roms/gba in order to avoid confusion with the gbc emulator. Also double joycons now work in unattached mode!
Merged gdkchans changes to the ppu. Some games that had problems with artifacts on screen (I tried harvest moon) now look much better!

Download http://www.nintendomax.com/viewtopic.php?f=168&t=16565