If im a machine gunner in the marine corp can i transfer to be a gunner mate?

By "gunner mate" I'm uming you mean the GM rating in the Navy. A friend of mine got out (0311) got out and tried to join the Navy as a Corpsman. They said no to being a HM and said he could be a Gunners Mate or Master at Arms. So depending on needs of the Navy I'd say most likely.