i generally agree with jordan's and the selected comments by gator as well.
MsPoly, if yours is a YOY (young of the year) alligator gar, it should have grown 12" in the first year...if not, that fish is stunted. now, keep in mind that doesn't mean the fish will grow 1" each month...growth before piscivory (predation on fishes) is not quite as fast, but once the gar switches to eating fishes and other similar prey items, growth is extremely fast.
how often are you feeding the gar and how often do you do water changes? sorry to derail, but these are basic issues which can drastically constrain the first-year growth of a gar. again, a healthy gator, even accounting for genetic anomalies should be around 12" at the end of it's first year if properly cared for. a good thing you have going in your tank (and reason why you're not seeing a ton of aggression yet) is that the tank is quite large...this will buffer aggression a bit. the mysteriously small size of the gator is the other big factor lowering aggression...most 12"+ gators are going to knock around gars of similar or smaller sizes, and should be well on their way to outgrowing them--
--solomon