They make poisons and traps for them.
Poisons can be bought at the farm supply stores. They also make a tool to put the poison seeds in the tunnels. But, you can take a hoe handle and punch around the mound of dirt and usually find a tunnel. Drop seeds into the tunnel and then cover the hole, made by the hoe handle. If you dont cover the hole the gopher will push up more dirt (they dont like the fresh air and sunshine) and not eat the seed.
Traps can also be bought at farm supply stores. Dig up the mound and you will find a tunnel or two. Place the trap in the tunnel as far back as you can reach (dont be afraid to dig the tunnel out by hand). Place something over the tunnel mouth to keep out most of the sunshine, we used dried cow paddies. Be sure to stake the trap because cats and dogs will tote off the traps and eat the gophers. It may take a few tries to get the gophers, but if you stay at it you can really put a dent in the population.
