G-Gundam

Note: This game is also titled Kidou Butoden G-Gundam.

Devil Gundam:
Highlight any robot, then hold L + R + Select + Start and press A at the character selection screen.

Burning Gundam:
On the second battle with Master Gundam, lose four times to receive the Burning Gundam.
Information in this section was contributed by Alex Roh.

God Gundam in story mode:
Lose the second battle against Master Gundam with the Shining Gundam and continue. The Shining Gundam should be replaced by the God Gundam.
Information in this section was contributed by Daniel Mainardi.

Note: If you cannot wait to get God Gundam, die a few times on any battle until you get the God Gundam.
Information in this section was contributed by Alex Roh.