Cheat mode
Pause in any game 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.

Cube Attack in Gradius 3
Survive until level 9.

Extra mode in Gradius 3
Survive until level 9 to unlock Extra mode.

Extra Edit mode in Gradius 3
Successfully complete Gradius 3 to unlock Extra Edit mode. Select Extra Mode to access more weapons.

Gradius level in Gradius 3
Intentionally get shot by the Final Boss to unlock the Gradius level.

Salamander level in Gradius 3
Intentionally get shot by the Final Boss to unlock the Salamander level.

Stage select in Gradius 3
Successfully complete Gradius 3.

Boss Rush mode in Gradius 4
Successfully complete Gradius 4 to unlock Boss Rush mode.

Stage select in Gradius 4
Successfully complete Gradius 4.

Stage select in Gradius Gaiden
Successfully complete Gradius Gaiden on the Normal or higher difficulty setting.