A fangay is the equivalent of a fangirl or a fanboy that refers to gay people (men usually) who are passionate fans of stereotypically "gay" popular culture and social cultures, activities, etc.
This word is not an offensive slur. It is not meant to be a synonym of the offensive way "gay" is often used.
It is often used affectionately in conversation by gay men, the girlfriends of gay men, and their boyfriends.
My friend Jack loves the show Queer Eye for the Straight Guy so much. He's such a fangay.
My GBF fangayed so hard last night while we were watching Glee.