Why is Wilson moving Myrtle out West in Chapter 7 of The Great Gatsby?

Wilson has discovered that his wife, Myrtle, is having an affair.  He does not know who it is, but it really does not matter.  His wife is being unfaithful, and Wilson thinks that moving out of the Valley of Ashes will fix the problem.  In all likelihood, moving Myrtle is a temporary solution.  She will no longer be having the affair with Tom, but she will likely still be unhappy with her station in life and marriage.  Moving Myrtle out west is not likely to fix the marriage.  Wilson may or may not know this, but he does know that moving away from the Valley of Ashes will stop Mytle's current affair.  It is a brave move on Wilson's part as well, because in some ways he is giving up on his belief that he can strike it rich in the East.  However, the move does show that saving his marriage is more important to him than making money.  

In The Great Gatsby, why does Wilson lock up Myrtle, planning to take her west?

A careful re-reading of Chapter Seven should help you identify the answer you are looking for. The appearance of Wilson with his "hollow-eyed" gaze and his general weakness certainly serves to illustrate that something is very wrong, and perhaps more than just physically with him. However, let us pay careful attention to Wilson's words as he explains why he needs some money quickly:

"I just got wised up to something funny the last two days," remarked Wilson. "That's why I want to get away. That's why I been bothering you about the car."

Thus we can see that he at least has become partially aware of the other life that his wife is living and has realised that their might be some form of infidelity involved in it. Consider the comment that Nick gives us after Wilson's speech:

The relentless beating heat was beginning to confuse me and I had a bad moment there before I realised that so far his suspicions hadn't alighted on Tom. He had discovered that Myrtle had some sort of life apart from him in another world, and the shock had made him physically sick.

Wilson has therefore discovered something of Myrtle's little secret, though he has not yet linked Tom with this secret. The shock has almost physically destroyed him, and has made him determined to move West quickly to take Myrtle away from the dangerous influences that threaten to tear his marriage apart.