I've always found live food to be the best for quick growth. Main reason is you can always have food in the tank that the fish will eat as it chooses. Whereas with prepared foods there is the risk of uneaten food causing water quality issues.
Finding a source of clean feeders and enough feeders can be a problem as well. My one tank will polish off 500 rosie reds in a week if I let them. Because of theis I would suggest switching to non live foods once the gar has some size on it, ex. 8"ish for a florida gar.