You need to update the RetroPie Setup Script first. To do so, change to the directory of RetroPie Setup Script,
cd /home/pi/RetroPie-Setup/
and update the script via git,
git pull
Then, start the setup script via
sudo ./retropie_setup.sh
change to source-based installation and select only the GBA emulator gpsp to be installed.
Afterwards, re-enerate the Emulation Station configuration file from the “setup” menu.
I have written this without explicit testing, please report any issues that possibly occur.
Hope that helps!