I've been doing the same, can't figure out. The LFS said it should get about 24". They have a lot of people working there and they all seemed very knowledgeable. My only concern is what to feed it, if it's not a "true" gar. They said pellets, blood worms etc.
Spotted Pike Characin (Boulengerella maculata). Not a true gar, but a predatory tetra from the Amazon...a bit trickier to get onto pellets than a true gar, you may have to use worms, frozen krill or fish, etc...