again, really sorry to hear about your gar
as for the post-break movement, don't be fooled by just vibrations/percussive movements from the front part of the fish causing the back to move. the back fins should be able to move in a fluid motion separate from the front...with a broken back the movement will be somewhat jerky or erratic. the other problem is that further movement will often times cause the gar even more injury.
i have kept gars around in the hopes that they will recover, the majority of cases they got worse and had to unfortunately be put down.
i am not saying this is definitely the case with yours, just something to be aware of and look out for. i had to put down one of my longnoses today that i had been growing out for several months...it had broken its back at the posterior part of the body and i gave it the weekend (even some food) to see if it could get better, but it just got worse and had no control of posterior fins.
keep us posted, and we're pulling for the gar to come through!--
--solomon