Enable SSH on the Retropie, will be easier to go back if needed
Edit /opt/retropie/configs/all/autostart.sh
Replace emulationstation #auto
with the following
ls ~/RetroPie/roms/gb |sort -R |tail -1 |while read file; do
trimfile=$(echo $file | rev | cut -c 4- | rev)
/opt/retropie/supplementary/runcommand/runcommand.sh 0 _SYS_ gb ~/RetroPie/roms/gb/"$trimfile".7z