Nuclear Gandhi, as far as I can tell, was actually about Civ I, not Civ II. Gandhi would get to nukes early because India had a tech boost, and he would threaten the player in every diplomatic interaction that his words were backed with nukes. The AI in the early Civ games is pretty aggressive, too, so I assume that if you rejected him too many times, he absolutely would become a big warmonger.

As to the integer underflow, that appears to either be debunked or to be something that Sid Meyer misremembered (IMO both are possible). I don't think you need an integer underflow for Gandhi to go nuclear in an early Civ game, though.