Green’s Dictionary of Slang

gates n.2

1. (N.Z.) the lips.

[NZ]N. Hilliard Glory and The Dream 76: Not with a snuffstick hanging out of their gates.

2. (US gay) the buttocks.

[US]B. Rodgers Queens’ Vernacular.
[US]R.O. Scott Gay Sl. Dict. 🌐.