Well . . . Gars are pretty much an "add water" fish. But they don't live very long if they can't come up to the surface to get air. . . So I'm sorry, but I can't really help much there. I know the LFS I work in has gotten maybe 8 shipped in through an overnight delivery since I started there, and all 8 of them arrived fine and did great.
I would think it would be OK as long as it's shipped in a big enough container with enough water & air in it, but I'd be nervous about the "standard" shipping methods where bags have 1" of air at the top of the water and that's it. I'd also worry about shipping it in a tiny container because if it gets scared it will spook & break its beak or back.