Just to nitpick. Those gamblers are not betting on the Gophers to win but rather the Gophers to cover. The next point is the big one. If the Gophers get within a field goal, it means a ton of money has been bet on the Gophers and they need MSU money to cover. By going under a field goal a lot of betters on the fence will join in.
6.5 was too many. I would say 2.5 is about right.