I'm pretty sure that a few people on here have bred gars before.
There was one guy who bred tropical gars, but he had them in decent-sized ponds and was located some where in Central America if I recall correctly. There was another guy in Singapore (?) who got his Florida gars to breed in his aquarium, but I'm pretty sure that he used hormones on them.
Solomon (E_americanus) and Richard (Pejelajarto) would know more about getting gars to breed than pretty much any other member on here.
Of the three mentioned above Tropical gars (TPG) would be the one to mess around with. Forget Gators and if you really want me to tell you why I will, but in short they are not an amateurs fish in any way and spawning is a case where this is even further exemplified. Platyrhincus have their own set of issues and problems also just by being a temperate water fish. TPG on the other hand seem to be extremely well suited to captive spawning in small systems.