Gate valves are not typical for this application, and if they truely are gate valves, the gate may have broken off, and the only way to resolve this is to replace the valve. My guess is that you've just given us the wrong name for the valve. On a true gate valve, if it fails, you can turn the handle continuously, and it won't move the gate, as the connection between the two can break.
When you turn the water off on an older system and drain the lines, when you turn the water back on, it can dislodge lots of potential crud from the lining of the water pipes. This can play hell with valves, showerheads, etc. Maybe post a picture of what you have...as it doesn't make a lot of sense.
At some point, all shower valve manufacturers were required to install a pressure balance valve in the assembly to prevent scalding incidents from fluctuating water pressure. It's possible that this valve is jammed in one direction, expecially if you turned on supply on and then tried to turn the shower on...they only work if the pressure is fairly even on both supplies. If it is additionally got crud in in, you may need to replace it (if it even has one - depends on how old it is).