Its already been mentioned, but a good rule to follow for everyone above you is to use the proper address until you are specifically told otherwise. Just because someone else refers to a person as gunny, top, master guns, maverick, douche-bag, or anything else doesn't mean you can.
If you called him gunny before, he might still opt for a less formal form of address now as well. In any case, he will let you know.
The same applies to your instructors in flight school. Your on-wing's name will be Lieutenant/Captain ____ or Sir until he tells you otherwise (which may never happen). The thing about formal address is that it naturally provides a little bit of distance, which can be a good thing.
As far as custom goes, I believe "top" normally refers to a Master Sergeant in the Marine Corps or a First Sergeant in the Army (not a First Sergeant of Marines).