I want to try my hand at mounting a muskrat. My question is can I freeze it whole without skinning and gutting? Because of my inexperience, when I get one I like, I want to plan it properly and I don't want to be rushed.

It depends on how long you keep it in the freezer. The nose and the lips and the toes will freezer burn after a while, and you will be SOL.
I would skin it and fold it up face in to protect the nose and face area. Then wrap it up in a paper bag or if you have a small cardboard box put in the box.

Why would you gut it? you're not mounting the whole carcass. You have to build a form or maybe you can buy a rat form from a taxidermy shop.

I guess it is assumed by leaving it whole it will still have the guts in it lol. I always gut the muskrats and freeze the meat for bait, so it is just habit to talk about gutting the animal.