Lucky for you gars breath from the surface and can tolerate being out of water for a much longer period of time than most fish.
I had a three foot gator gar jump out of an indoor pond I was keeping him in while I was at work. I had to rush home from work (about an hour drive) to throw him back in the water as my housemates at the time were to freaked out to do anything about it. The fish sat on the bottom of the pond for an afternoon and he was back to normal.
The only thing I would mention is that their eyes can dry out pretty easily and that after my guys jump his eyes always maintained a slight haziness. Lucky you had some water that he found his way into!
My suggestion is to throw another oxygen stone in there and do water changes every day . smaller ones but consistent for at least two weeks the drain is gross !!