yes Alligator gar require a couple thousand gallons however my gar is more or less a slow grower. .... I have been told that it is odd that he is close to 5 years old and less then 3 feet. I have always had him in tanks with ample space and filtration but I keep the gar at a colder temp and I feed once a week to keep him a little more lean...
I've had two gator gars for a long time, lost them at about 7-8 years of age both at 3' exact. My keeping conditions mirrored yours.