Hey there,
I’ve written a setup script that can resize your file system and partition to fill the entire SD. It works on the fly and you don’t even have to reboot 
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)
To see what else it can do or look at the scripts themselves, check out the repo: https://github.com/hpcodecraft/gameshell-setup