I don't know how similar your gophers are to our pocket gophers Pocket gophers seal up their hole. The only way we can manage them is with traps. You buy these nasty-looking spring-loaded traps for $4-5 at the hardware store. You need 2 for each gopher hole. Dig out the dirt that is covering their tunnel. Dig until you find the fork in the tunnel--where it branches into two tunnels. One is a false tunnel, but since you don't know which is which, you put one trap inside each tunnel. The trap should have a chain or wire attached to it so that you can anchor it down at the mouth of the hole (we use a big 4" long nail to secure the traps.)

When the gopher senses the cold fresh air, he comes up to refill his hole and gets trapped. You come out 1-2 hrs later and voila'--dead gopher. If you come out and find the hole filled in, he managed to avoid your trap and you have to do it all over again.

Depending on how prolific they are dh can spend 1/2-1 hr a day, 2-4X/wk setting and emptying traps. We have 8 traps, so we can go after 4 holes at a time. An average week there's only 2-3 holes but sometimes there is an invasion and 5-6 will show up overnight. They are nasty creatures that--like yours--did not respond to poison pellets, flooding or gas fumes. Trapping is the only thing that controls them. We've been able to keep them isolated to small segment the property. If they go much further they could be chewing into underground electrical or phone wires.