What causes fish to smell are mainly two chemicals, dimethylamine, and trimethylamine. They are broken down from trimethyl oxide, which is present in the flesh of all fish. When a fish dies and becomes exposed to oxygen and nitrogen, it's own bodily enzymes and bacteria begin to break down the trimethyl oxide into dimethy and trimethyl amines, which are very volatile, and very reactive to nasal receptors. This is probably so that scavengers can find it quickly and dispose of it, returning it back into the system.
Gar have no more amines than any other fish. It may be that gar carcasses are just left out more often, and for longer periods. They have an outer hard plating of ganoid scales that may discourage some scavengers until they soften up a bit.