I would post some pictures up first and let other members here see how bad it is before deciding on whether or not to put the fish down... unless its obviously in a lot of pain and will never be able to eat again... then it maybe more humane to put it down instead of let it suffer a slow death...
Try and see if it can still eat.. If it can't.. then its time to put it down.. The chances of it being able to eat is slim though.. Best of luck to you.
Post this over in the "Gars & Bowfins" sub-forum; there are a couple gar experts over there that can provide you with the advice that you're looking for.