How big is it now? That's the first question that needs answered before making the choice to buy (or not buy) the gar.
Anyway, I can tell you right now that a YOY (young of the year) "spotted gar" will outgrow a 55 gallon aquarium reasonably fast, but a small one (<6") might be able to get a few weeks/months use out of it if grown-out in a smaller aquarium (usually a 20 gallon long) prior to moving it to the 55 gallon aquarium. You'll still need to move it to a larger aquarium in less than a year, but you'll have more time to get a larger aquarium than if the gar was on the larger side for a YOY gar.
By the way, I say "spotted gar" rather than spotted gar because you mentioned seeing the gar at an LFS (local fish store), and they often incorrectly label Florida gars as spotted gars. Of course, this is barring the situation in which the gar was collected locally in which case it is a spotted gar in all likelihood (unless the LFS is really bad at IDing gars, that is).