I'm not a pro, but a gate valve is probably the last thing I'd want burried in the wall...a ball valve is much better. A gate valve may not work down the road when it is needed. There are even smaller ball valves that instead of a handle, have a screw slot that you turn which take up even less room. Note, while handy, shutoff valves are not that common in a shower. First, you have to know they are there, and second, many years down the road, they may not work all that well if you do use them. A good ball valve should last a very long time, though. Often, the shower valve itself can be purchased with or without shutoffs...those are designed to be able to be used for maintenance after taking the trim piece off.