Most likely the "spotted gar" at your store is a Florida gar (both have similar growth anyway). There are a few factors that come into play when talking about growth rate but generally speaking 1" a month can be pretty normal for spotted/florida gars from a young size up to about 18"-20". Some grow as quick as 2" a month when under 12". Sometimes they grow slower than that. For the most part, it will vary from individual depending on its genetic and the care provided (water quality/water changes, diet, how much it eats, tank size).