Is the best way to remove the “Gameover” virus/ransomware to install Avast and do a full scan followed by removal by Avast? Is it necessary to run Avast from a bootable USB stick and is this even possible?
To my knowledge there is no bootable scanner for the Mac in this way like for windows. Have you installed avast to see if it can detect it?
Are your files encrypted, and do you have a good backup to restore from?