To the OP: they will survive as long as they are eating.
My first FL gar died of a broken back / stress in my 180 (I believe my Odoe tortured him) - he was about 12"
My second gar (an alligator gar) died because he starved himself after he broke his back (again, my Odoe was likely the cause) - he was about 14"
My current gar (who I ordered as a completely different fish, but definitely grew up to be some sort of gar) has a broken back thanks to my African tigerfish, but eats every night. He was moved to a 40br with a lone feeder goldfish who's too big for him to eat. He's about 5-6" long. I am currently struggling with whether or not to euthanize him, because he looks completely miserable when he tries to swim, but I can't bring myself to do it because I've had this fish since he was about 2" long. so for now, he can have the 40 gallon to himself.
As you can see, I have had zero luck with gars, and I will not be purchasing another one since my stock does not seem conducive to their well being.