I've never done this with gophers, but I've done it with moles and if you can do this safely, it works great.

Pour a good amount of gasoline down their hole. If it is a large networked hole, even better. Pour a few gallons down if the hole will hold it. In about a week, most of them will be dead or driven away from the fumes. If that doesn't work, then for round two, pour a few gallons down 3 or 4 holes that are really close and wait about 10 minutes. Then just toss a match down one of their holes (remember fire safety!). Enjoy! If you are really intent on shooting them, then pick them off as they scurry out of their burning and smoking holes.