Sounds like you've got a pretty good-sized pond and not enough fish for it. Get some more SMBs, bluegills, and a ton of shiners for in there; let those breed for a few years then add the gars once they're big enough to go in. You might have to tinker with the amount of shiners you put in so that you can maintain a reasonable population, but they shouldn't all get killed off too fast if they're given enough hiding places.
My four largest gars (2x longnose gar and 2x alligator gar) are in a slightly smaller pond (40' x 80' with the depth varying between about 2' and 8'), and they're doing great with the previously mentioned stock.
My four largest gars (2x longnose gar and 2x alligator gar) are in a slightly smaller pond (40' x 80' with the depth varying between about 2' and 8'), and they're doing great with the previously mentioned stock.