Welcome to the FX codes   Blue codes are old, they can be found everywhere or in a few places.   Red codes I've found in mags and sites but I can't get them to work. (they are probably fake)   Green codes- I don't have this game so I can't test it.   Orange codes are discovered by me, you might find them elsewhere but they were here first.
GAME GEAR
Sonic 1
Blast through Green Hill Zone Act 1 - When you see invincibility, skip it, go down the hill, get the power shoes, go back up the hill, get invincibility and roll down the hill and hold right