Not that I advocate the distruction of small fury critters that burrow underground and destroy gardens and layouts, but, I curred a gopher problem once with left over cans of foam insulation. I found the newest gopher hole (the one with the freshest pile of dirt around it) emptied the remaining contents of the insullating foam can into that hole. found the older holes and did the same. Then I waited twenty four hours and looked for new holes,,, none appeared, so I pulled on the cured foam that was popping out of the ground like a mushroom and lo and behold, up from the newest hole emerged not just the foam but embedded in it was a gopher, up to his hind quarters in foam. One of the other holes had a small gopher in the foam, but I imagine the rest of the gophers either got the stuff in their eyes and mouth and died somewhere in the tunnel system, because I didn't see hide nor hair of them again.
Cruel yes, safer than gas, definately