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I ported the gameboy advance emulator gdkGBA (originally made by gdkchan) to the nintendo switch!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.

UPDATE:
Sound now much better now! Many thanks to @renhei for the fix!
UPDATE2:
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!
UPDATE3:
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