I agree completely that it doesn't make sense that they wouldn't play Gurley more. And the truth is, I had the same initial reaction you did - that Gurley must have been injured. But when I started looking for something to confirm that I found that all the evidence indicates he was not injured. I'm as baffled as you are.
As for Anderson, he's been huge for the Rams since he signed with the team, putting up big numbers in most of his games as a Rams. But they didn't use him either. Only 7 carries.
It seems McVay didn't have any confidence in the running game despite how successful both Gurley and Anderson had been running the ball for the Rams.