I killed almost 700 of them over a couple year period at my old house (sandy soil). I may need to do a name change to the Gopher Whisperer.
First, gophers put up big mounds and the tunnels are typically 12-14" underground (especially if soil is very easy to dig in).
Moles have a tunnel just below the surface and it pushes the grass / earth up along the tunnel.
Assuming you have gophers... find a mound and start digging around the mound (not directly on top of it). You will find their tunnel (usually on the side of a mound leading toward another mound). Within 15 minutes, the gopher will show up and try to close the tumnel. Important - they will NOT leave a tunnel open more than 30 minutes or so.
Once I found the tunnel, I would dig a long sloped hole, sit in a lawn chair about10' away with the 12 gauge and blast at an angle where the bb's traveled nearly parallel into the tunnel. I would dig and find them mutilated 3-4' back into the tunnel. If I didn't find them, I usually gave up on that area and opened up another mound because they wouldn't come back to that one for days (if at all)... they would seal it off.
Alternately you can do the same but place the victor trap in the tunnel after digging it out... but be sure it has a chain that is anchored, because if it doesn't cath them in the ribs, they will drag the trap 10' or more in the hole.You need to put the victor trap in the hole where the flap (trip) is right at the opening. You also want to sprinkle a little dirt to hide it a touch.
I will admit, it was a lot of fun... but they are smart... if they sense you are there and you don't shoot them at first sight or they know the trap is there... they go back 2-3 feet into the tunnel and seal it off.
So, after 15 minutes, if you don't see it again, start digging and you will find they packed the tunnel with about 12" of dirt extremely well to close it off. Keep digging, reopen it and try again.
If you dig a 5' diameter hol, 3' deep, they will try to fill the entire thing back in. Once you see dirt flying out of the tunnel, take aim, hold real still and when you see fur, blast away.