Gophers can be such a pest! One of the better investments I made for one of my current gardens was to bury 1/2” hardware cloth 18” down around the whole perimeter. I still have to deal with the occasional gopher in different areas though. Bill Murray had it right in the movie Caddy Shack going all out war on the darn rodents with C-4!
My standard process in dealing with them at the first sign of a new mound is to first excavate the new tunnel they made evidenced by the new mound. I do such using a soup spoon, prodding to ‘feel’ the loose soil to determine their back-filled tunnel. I then will place a trap in the excavated tunnel and use a piece if fruit, such as a slice of apple or pear, behind the trap so that the gopher has to push through the trap (and get caught) to get to the fruit. I have a couple dozen of this type trap: http://www.victorpest.com/store/mole-and-gopher-control/0615
The second method I use if the gopher is too smart to get caught in the trap is to once again excavate the tunnel, and then place about a spoonful of gopher bait in the tunnel, covering up the entrance with a rock or something so that the gopher doesn’t just sense that it’s tunnel has been opened up and immediately back-fill, bypassing the bait. I use this brand bait with good results. http://www.solutionsstores.com/mobile/Product.aspx?id=41092
If neither of those two methods work, I then have to go out as soon as it is light enough to see, look for the recent activity, see if it’s hole is still open, which usually means it is not done yet and will shortly be back to back-fill the entrance, and wait patiently with a firearm pre-aimed at the hole. That ALWAYS works but requires patience.
I’m not sure what type snakes you have in your area, but by me there are bull snakes and gopher snakes. I keep a grain bag in the truck and if I see one crawled up on the highway in the evening (soaking up the days residual heat) I’ll stop, catch it, put it in the grain bag, and let it go down a gopher hole near my garden.
I know some people who will only kill the gophers that are actually in their garden. I go further out in the surrounding area since they seem to eventually make their way into the garden. One other method I’ve heard of with good results is to gas the gophers with CO2. I had on friend who was a bit of a pyro who had a blast using Oxy/acetylene pumped into the tunnels and then igniting it....
https://www.youtube.com/watch?v=hQbZ4Z_3uXU