Gatling guns would certainly have helped, but they may not have been sufficient to pull off a true 'win' given teh numerical disparity.
Generals do typically get credit when their side wins (even if it's in spite, rather than because, of them), so there would be nothing odd about Custer getting credit. However, it wasn't a big enough expedition/war to really warrant that turning into political fodder.
Custer was fairly well known for his exploits in the civil war so he could have pursued some level of politics if he wanted. However, I'm not sure his tempermant would have lent itself to slogging through politics. If he did, he could be a congressman, governor, etc.
If he was elected president, the success of his presidency would depend on who he appoints to his cabinet and on luck (as all presidencies do). It would appear likely that he'd appoint some military cronies and such, so (if not careful) his presidency could wind up a bit like Grant's