Replace the screen if screen rot occured, check all capacitors for signs of bulging and replace those affected, if you are planning to get the battery pack, replace the cells if they no longer keep charge, finally check the state of speakers and replace if necessary. Tuning work would consist of swapping the membranes under buttons if they show signs of decay for new ones and cleaning the connectors underneath to improve their responsiveness as well as the connectors in the cartridge slot... And that would be all, I think. For some reason, I find in-depth modding like case jobs, adding switches or custom painting to be more damaging then beneficial - it takes away the console's identity. When preserving a Game Gear, make sure it looks like a Game Gear on first glance.
