The only people I know who spawned North American gar did them in earthen ponds outside. Ponds were roughly 50'x50'x4'. As I understand it they just stuffed the pond with feeders like gambusia, mollies, and other small fish. Then they spawned naturally in the spring. I don't think the ponds had to be that big but that is what they had.
At the risk of asking offensively stupid questions. When you are using hormones are you sure they are ready? Are you using a catheter to sample eggs and checking for nucleus migration? Hormones don't force fish to spawn, they can only take a fish that is ready to spawn and push them over the hump to bypass the final triggers for spawning.
Also are you going with in vitro or injecting and waiting for a natural spawn afterwards like LSU in the video below?
Gar Spawning - YouTube