The Nintendo code, which worked exclusively on Konami games was:
"Up, up, down, down, left, right, left, right, B, A, Select, Start."
This code gave you all kinds of goodies on various Konami games, but it didn't work on all of them.
I agree that G&G may be the most impossible game to beat ever made. I think I may have beat it once, or at least gotten to the last level. In the arcade, I never could seem to make it through Rampart. Also, the D&D quiz game was damn near impossible - I once spent $6 trying to beat it, and didn't even come close. Actually, the other D&D arcade games were incredibly hard, as well.