In American English the name gar (or garpike) is strictly applied to members of the Lepisosteidae , a family including seven living species of fish in two genera that inhabit fresh, brackish, and occasionally marine, waters of eastern North America, Central America, and the Caribbean islands.
That's the problem with common names, everyone thinks theirs is the only correct one. The only real names are, depending on the species:
Lepisosteus oculatus Winchell, 1864 (Spotted gar)
Lepisosteus osseus (Linnaeus, 1758) (Longnose gar)
Lepisosteus platostomus Rafinesque, 1820 (Shortnose gar)
Lepisosteus platyrhincus DeKay, 1842 (Florida gar)