Tail-Gunner is a slang term for a male homosexual.
The origin of the term should be self-explanatory.
The metal band Iron Maiden have a song by the name Tail Gunner, but it is about a man who shoots a machine gun from the tail of an aircraft. What a strange notion.
In the US the term Tail-Gunner is also used to describe the act of urinating whilst clenching one’s penis between one’s legs pointing backwards.
“That tail-gunner keeps checking me out, but I don’t fancy him one bit!”.