Sonic Gems Collection Cheats

Select Sonic as your character in single player, and fight your way through the 8 battles without losing a single round (that means no losing the match and starting over from round 1). Defeat Metal Sonic in stage 9 in round 1. In round 2, activate Hyper mode (back+punch+kick) and Sonic will transform into Super Sonic, making him invulnerable for the rest of the game.

If you have Radiant Emerald and Super Sonic unlocked, just select Super Sonic and play as him in Radiant Emerald to hear him theme song, Super Sonic Racer, instead of Diamond in The Sky.

To get a Game Over without dying, don't let Sonic move, jump, ect. for 3 minutes, then Sonic will say "I'm outta here", and you will get a game over.

When examining an illustration in the Museum, press X. You should hear a sound. A slideshow will start featuring all the pictures you've unlocked so far. It'll skip over:

Non-pictures (videos, demos, etc.)
Pictures still marked as NEW
Pictures you haven't unlocked