I originally misread an earlier post on my way out the door and thought that no one had been able to get into the debug mode.
See my last post for the invincibility trick.
Keranu, at the intro just hold Select down, then press Left then Right alternately (10 times) until you enter the Sound Test.
EDIT: I just got the trick to work. You're supposed to reset once the game crashes and then start a just game and you'll have invincibility. Select + Run wouldn't work, so I just used Magic Engine's reset feature to get it to work. Maybe on real hardware you need to quickly turn the power off and back on to do it. I'd try it myself, but I don't own Gunhed and am too lazy to put it on my flash card.