Lyric discussion by musicmax 

Cover art for G.P.T. lyrics by Martha Wainwright

I believe it stands for "Greenpoint Tavern" which is a pub in Brooklyn, New York. I guess it would be the setting for all of the stuff going on in the lyrics.

And yeah, I agree with azkm, the song is about libido, promiscuity, the carefree and decadent hedonistic party life. See: Great Gatsby.