He may heal, he may not. It's unpredictable at this point. I would just give him a tank by himself to rest. I can't tell you what will happen if he doesn't heal soon, but my guess is he'll remain like that for life. But I could be wrong.
they will grow if the injury doesn't affect their life that much. I've seen a 5.5 feet(167cm) with broken back at Texas freshwater fishery center, and I don't think a big gar would break its' back like a young gar since their spine is stronger and their control over the speed is a lot better.Sorry to hear about your gar, I have a spotted gar that is about 15cm that broke his back but the other way round its like a "U" shape, he´s alive at the moment and broke his back 1 year ago, he eats, he swims around with the other gar that is 35cm and is perfectly fine apart from how he swims. I think you should wait and see. Also gars after breaking their back won´t grow any more, my 15cm gar is older than my 35cm gar.