Hiya,
The true spotted gar is Lepisosteus oculatus, a long lived gar that reaches almost 4' long but most of the time stays closer to 3'. I believe it is the widest spread of the gars as it is still found in isolated pockets as far north as Toronto, Canada and in Lake Erie all the way down to Florida. It is tolerant of temps down to near freezing and up into the low 80s, it also tolerates brackish water but is more common in fresh.
The boullengerella maculata, south america's spotted pike characin is often sold as a tropical spotted gar but only reaches a little over a foot in length, it is similar in appearance, so much so that fishbase has a picture of them mixed into the ID photos of the real thing.
The upper shot is the spotted gar, the lower one is the characin.