"In the original Civilization, it was because of a bug. Each leader in the game had an "aggression" rating, and Gandhi - to best reflect his real-world persona - was given the lowest score possible, a 1, so low that he'd rarely if ever go out of his way to declare war on someone."
Heh.
Source:
http://kotaku.com/why-gandhi-is-such...ion-1653818245