If you leave the game on for more than 50 hours (yikes!), you can select differently colored versions of Dunbine and Walker Galliam by pressing Start on them on the select screen.
Play as Devil Gundam
Beat the Story Mode with all characters, then use use Master Gundam in the 64Mode and beat Devil Gundam. You can now select Devil Gundam
Play as Judecca
This one's a bit insane: If you don't switch your game off for 300 hours -- we're not kidding -- the strange insectoid Judecca will appear. Happy heating.
Play as Master Gundam
Beat Master Gundam three times in the Story Mode, and you will be able to select him (Story Mode only).
Super Mode Shining Gundam
Use Shining Gundam in the VS Mode and win 20 times, Super Mode Shning Gundam will appear the 21st time. After this, go to the select screen and press Start while selecting Shining Gundam. Now you can use him everywhere, including the Training and 64 Mode.