There is only one true definition,

Adj. Something that is expected to have a long lifespan by virtue of having existed for a long time, based on the lindy effect, named for the New York restaurant Lindy's, coined by Albert Goldman and popularized by Nassim Taleb.

When something is really gay looking like a gay post, meme, song.
Favourite thing of {LGBT+} community to say when going through their daily routine of gay.

An act or a comment by a person that has queer tendencies, but is not necessarily performed by a homosexual.

It can't get much simpler than this: Shit that is gay.