Welcome to the AA forums! I think you'll enjoy yourself here.
There were two different G.I. Joe games released for the NES -- which one did you find? (Inquiring minds want to know! ) If it was the original Taxan release, and not "The Atlantis Factor" from Capcom, you sure lucked out -- that one is VERY HARD to find in the wild, and for good reason: that game is freakin' awesome!
It IS the Taxan release! Been having good luck at local Salvation Army stores lately. Lucky Me!!
And thanks for the welcome--I've been lurking here at AA for about a year, just decided to start chiming in...