I am curious, how do Marines react when another branch calls them "Gunny"?

Well, in my experience, if you are being told to report to the "Top" or the "First Shirt" your a** is grass and the "Gunny" already mowed it! :-)

Sgt Aaron Kennedy, MS
Sgt Richard Buckner

LOL, in some way that is true of all services I think GySgt Wayne A. Ekblad!

Yes, you have to be very careful with Top. Not all like being called Top. If they don't like it and you use it you will not like the outcome.

Cpl Jeff N. A great phone game was "MSgt Ramen called, needs you to call him back at X" and see who falls for it. "Devil, you're trying to get a hold of Top Ramen?!?"

Semper FI works too doesn't it Gunny?

As a greeting, of course, but I probably wouldn't call somebody that CSM Michael Lynch! ;-)

LOL of course not I would have added Marine on the end.

probably the same way AF folks do when they get called Chief.....shrug it off and press on....kind of like saluting on a Navy base....when in doubt throw it up.  

I never called a Gunnery Sgt anything other than Gunny. It is the unofficial rank title of an E7. Never met one that objected and I knew quite a few of them.

1LT Nick Kidwell with a First Sergeant, it's kind of like someone addressing you as "El-tee". It's a familiar name that you'd probably be totally fine with from someone you have a relationship with, in an informal setting, but probably not something you want to hear being called out by a random soldier without regard to setting.

SGT Richard H. - Makes perfect sense to me. The one who corrected me was one I had just met. He was coming in, our old 1SG was going out.

SSG Dr. Roy Coble - Why would you interpret it as a pissing contest? I'm not getting that vibe here.