imo a shallow tank, and try and keep him upright.. he needs to be able to breath air as well as breath in the tank. the problem is the stress.. gar can go a long time w/out eating when adults. I actually disagree... if the fish was going to die from its injuries it would have succumbed already... as for the back healing properly its hard to say, once the swelling goes down you will know if its capable of swimming on its own again, but that could take weeks or even monthes depending on how severely it was inflamed/damaged. my FLG had a similar instance not as severe but it took a few weeks before he came around compleately, and looking at him now you'de never guess. I'm guessing the paralisis is from the inflamation and not neccisarily the initial accident as he was "ok" right afterwards.
idk if you can give anti-inflammitorys to fish.. but it would be something I would look at.