Giga Wing Cheats

From the main menu, select Gallery and press:

B, X, Y, B, B, Y, X, B

You'll hear a sound effect if done correctly, and when a new game is started, you'll find a brand new ship available.

Choose the Two Player mode, have Player 1 pick a jet. Then hold Start and press A on the second controller when picking a jet for player 2.
Player 2 will be invincible for the whole game.

When selecting your player, instead of pressing just the A button, press either X+A or Y+A simultaneously to get two extra alternate colors.