Gex
Stage Select:
While in the level selection dome hold R1 and press Select. Then hit X, [], X, R, U, L, O, O, D, D.
Access All Levels:
Go to any stage and hold R1 and press Select. Then hit (), Start, R, U, [], L, L, U, Start
Codes:
Pause during the game, hold R1 while you enter the code.
Ice Shot: O, O, L, D, O, U, R
Super Jump: X, O, U, U, D, R, R
Instant Speed: D, P, R, R, D, U, P
Flame Shot: X, U, R, U, R, R
Invincibility: X, [], D, D, U, D, R
100 lives: U, O, /\, D, R, [], D
Electric Fire: R, L, R, O, /\, R, O, D, R
Goal Storm
Easter Island Heads: At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, Square, Circle. You'll hear the crowd roar. When you start the game all your players will have stone Easter Island heads.
Gunship
Invincibility: Start any mission and at the "Loading" screen, press and hold L1, L2, R1, and R2 simultaneously until the mission starts. The word "Cheat" appears in the upper-left corner of the screen.