To unlock all weapons and power-ups, pause the game while playing on the easier difficulty setting, then press UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, L, R to unlock all weapons and power-ups. In the Japanese version of the game, press UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, X, CIRCLE.

Get a couple power-ups and save your game, then load your game and you will notice that the power-ups that you earned are still there. So, if you repeat this, you will eventually have a full arsenal.

Unlock the following bonuses by performing the corresponding tasks:

BONUS - HOW TO UNLOCK
Cube Attack in Gradius 3 - Survive until level 9.
Extra mode in Gradius 3 - Survive until level 9.
Extra Edit mode in Gradius 3 - Beat Gradius 3. Select Extra Mode to access more weapons.
Gradius level in Gradius 3 - Intentionally get shot by the Final Boss
Salamander level in Gradius 3 - Intentionally get shot by the Final Boss
Stage select in Gradius 3 - Beat Gradius 3.
Boss Rush mode in Gradius 4 - Beat Gradius 4 to unlock Boss Rush mode.
Stage select in Gradius 4 - Beat Gradius 4.
Stage select in Gradius Gaiden - Beat Gradius Gaiden on the Normal or higher difficulty setting.