Cheat mode:
Pause in any game under their easier difficulty setting, then press Up, Up, Down, Down, Left, Right, Left, Right, L, R. All weapons and power-ups will be unlocked.
In the Japanese version of the game, press Up, Up, Down, Down, Left, Right, Left, Right, X, Circle.
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Gradius Collection Cheats, Codes & Walkthrough/Guide/FAQ for Sony PSP from Cheat Code Central
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Gradius Collection Cheats at CheatingDome.com.