This is a great resource for new GM's! My husband and I started doing Pathfinder with our three teens from the Beginner Box kit and loved it. When we were ready to move from the B.B. we bought the Core Rule Book--which is a great book but for a beginner GM there was a lot of "assumed" knowledge and I had more questions than answers when trying to run our games. I was excited to find the Game Mastery Guide because it simplifies a lot of concepts and goes into more detail than the C.R.B.
I also love the worksheets at the back for running a campaign, keeping track of settlements and NPC development. Great tool for this new G.M. who had multiple pieces of notebook paper with notes all over for a new campaign I was designing!
I think that more experienced GM's probably won't get a lot out of this book if they already have the C.R.B. but it's still a great product!