Johnny G-String
Hello.
In an American movie, which is called Click (2006), Michael "skipped" ten years of his life with a "magical" remote control. He doesn't know what happened during those years. He wakes up in an apartment and decides to go to his old home, where he finds his wife, Donna, with another man (Bill), and he seems to know Michael pretty well. Though, since Michael has no idea what happened to his life during the past ten years he acts as if Bill is a complete stranger.
Bill after kissing Donna: Donna and I were going to get a cup of hot chocolate. You want us to bring you one?
Michael to Donna: You cheating on me? With Johnny G-String over here?
Bill: Come on. Time-out. I thought we were past that.

Bill is portrayed by Sean Austin (https://en.wikipedia.org/wiki/Sean_Astin). It's winter and he wears a coat. So my question is, why on Earth does Michael call him "Johnny G-String"? Could you explain it to me, please?

Thank you.

EDIT: a picture of Bill and Donna: https://ibb.co/zn6DWFv

Question

Johnny G-String
Hello.
In an American movie, which is called Click (2006), Michael "skipped" ten years of his life with a "magical" remote control. He doesn't know what happened during those years. He wakes up in an apartment and decides to go to his old home, where he finds his wife, Donna, with another man (Bill), and he seems to know Michael pretty well. Though, since Michael has no idea what happened to his life during the past ten years he acts as if Bill is a complete stranger. 
Bill after kissing Donna: Donna and I were going to get a cup of hot chocolate. You want us to bring you one? 
Michael to Donna: You cheating on me? With Johnny G-String over here?
Bill: Come on. Time-out. I thought we were past that.

Bill is portrayed by Sean Austin (https://en.wikipedia.org/wiki/Sean_Astin). It's winter and he wears a coat. So my question is, why on Earth does Michael call him "Johnny G-String"? Could you explain it to me, please?

Thank you.

EDIT: a picture of Bill and Donna: https://ibb.co/zn6DWFv

Answer

A G-string (you can google it!) is something that most American men probably wouldn't use, but male strippers might. So Michael is saying that Bill is the type of man who would wear a g-string, not serious husband or boyfriend material. The 'Johnny', being such a common name is a way for Michael to further insult him, insinuating that he isn't anyone special.

Answer

I would have to watch the movie to really know for sure. But Johnny is a generic name and sometimes can be used to refer to anyone. G-string is a piece of revealing clothing, thought to be odd and distasteful. Simply put, Johnny G-string is an insult, especially if that guy has worn a g-string in the past.