If I'm not wrong, there isn't any way to tell if a captive gar is male or female just by looking at it.
Don't worry, spend enough time on the forum and you'll start to find it easier. Id'ing gars can be tricky initially, but when u compare them to the number of species AND within species variations other fish have (eg: polypterus, cichla, etc), there really aren't that many species' of true lepisosteidae.
I have a 21" florida gar. Been that size for a while. I dont think it will get much larger then it already is. Ive kept alot of florida and spotted gar over the years and this is the biggest one i ever had or even seen in a home aquarium.
Biggest one i've seen was at the lfs.. and i woulda grabbed him if the pool had been set-up already he was easily in the 24" range. by far the biggest florida i've ever seen. mine are the oldest and smallest gar in the pool except the trops.
I have a few Florida Gars, the largest is 23", the other two are 21" and 20" respectively. I feed them frozen shrimps and fish but occasionally live bullfrogs and tilapia. I also have one Longnose Gar at 20". They all co-exist peacefully.