The Gangers will lose and your Ganger rep will only go down to 'shunned'. You can run around and offer yourself as a target, saving a few Goodsprings lives, but don't fire a shot yourself. The trick is to organize Goodsprings and let them do all the shooting. I've learned that when doing the Ghost Town Gunfight quest, siding with Goodsprings, if you fire a single shot during the fight you will become automatically 'villified' by the Gangers and you won't be able to do their quests.
Some game designers didn't think that out well. You don't get the opportunity to raid the prison with the NCR unless you start doing the Powder Ganger quests first.
