I made a script to expand the file system on the fly, you can run it by connecting to your Gameshell via SSH and issuing this command:
bash <(curl -s https://raw.githubusercontent.com/hpcodecraft/gameshell-setup/master/run.sh)
The script can do some other things like updating Retroarch, too - see the repo for more info: https://github.com/hpcodecraft/gameshell-setup