Yep, it’s F10. The current GBA emulator (GPSP) is separate to most of the other emulators, which are all based on Retroarch and thus use the same control scheme.
If you have a Pi2, I would suggest switching to a Retroarch emulator for GBA.
To do this:
Enter the following into the console:
cd RetroPie-Setup
sudo ./retropie_setup.sh
then Update the Retropie setup script and restart it
Then go to Experimental
Install the GPSP Retroarch package. This will take about 20 minutes.
This will give you a new folder in your ROMs directory where you can put GBA games.