If you've got gopher problems, you probably have already tried many of the things I'm about to mention and know they don't work. Gophers don't frighten easily and all those devices would probably be enough to drive us away, just don't bother Mr. Gopher.
Flooding their tunnels doesn't work as you also know--they are good swimmers and can easily plug a tunnel up anywhere along its length when the water starts getting too deep. They do the same if you try to gas them.
Poison is an alternative, but not recommended. Before they succumb to the poison, they'll often emerge from the tunnel and die topside. Their carcass could then be consumed by a favorite pet and which would then poison them as well.
The best remedy is traps. If you're not too squeamish, the lethal variety, if done properly, will do the job. It may take several tries getting it set just right to nab them. Even though they are usually solitary critters, during the spring, they are usually in an amorous mood and will probably have a mate or two holed up with them so you'll have to keep at it putting traps in at several tunnel openings.
Find the main tunnel by locating a fresh opening and then determine it's direction by running a rod (or buy a gopher probe--really!) into the ground around an imaginary circle. You'll notice a give in the pressure when you hit the tunnel.
The main tunnel runs underground for a considerable distance. They will then dig up to the surface in various places and these are the mounds you see in your yard. You want to find the main tunnel, not the lateral access tubes.
Once you find the main tunnel, dig a hole that intercepts this tunnel. Set two traps, one on each side of the exposed openings so you'll be catching them either coming or going. Normally, you don't have to bait the traps, but some folks claim this helps. If you opt for this you might consider using lettuce, carrots, or slices of apple. Tie a wire to the trap so you can easily remove it. Once the traps are properly set, place some cardboard over the hole you dug and cover with soil so no light enters the hole. If you don't catch one in 3 days, you've got a dead-end tunnel and will need to relocate it and try again. Once you get the problem under control, be prepared for a re-infestation, especially if you live in an area bordering a vacant lot, or wild-lands. You might get rid of the ones in your yard, but as soon as that tunnel becomes vacant, another opportunistic critter will take advantage of the already excavated new digs.