This is another fairly dangerous suggestion, but it should be extremely fun
I heard about it for ground hogs, but I imagine it would work for gophers, you may have to do it a couple times at different holes though.
Anyway, you'll need five gallons of gas or so, a funnel and a very long hose. And a lighter or matches.
Step one: feed the hose into the hole as far as you can. Then reel it out so your as far from the hole as possible, it helps if you uphill also.
Step two: funnel the gas into the hose. Let it sit for a while to make sure it has run all the way down into the hole, and has had a chance to fume up. At least with ground hogs, they won't come out of teh hole if they know something or someone is near by.
Step Three: Hold the match up to the end of the hose, one of the long household lighters or a burning stick might be better.
The result should be a large explosion, in the hole that will push the ground up all the along the tunnels and cave it in. Should also be very lethal for any critters in the hole at the time.
Or, another idea
Take a look at all the holes you can find. Pick the one that you think is being used the most ( ground wil be dug up going in and out, grass or plants wil be smashed down at the mouth, fresh dirt etc.) Now, drop smoke bombs into all the other holes, get them down into the hole as far as possible, and cover them with something if you can. You want as much smoke building up in there as possible. Once thats done, camp out at the mouth of the hole that is well traveled with a shotgun .22 or any other weapon you choose. Be prepared for them to be moving quick if they come out at all.