Similar to going out on the "pull" or "pulling" somebody.
Going out on the "gull" or going "gulling" refers to somebody going out and picking up scraps, much like an actual seagull, of somebody's recently ended relationship or trying to sleep with somebody who is really broken, emotionally unstable and rather promiscuous.
Christopher: Broke up with Holly last week
William: Ah, man. That sucks. Watch out for Charlie though.
William: He is definitely gonna be on the gull.
Christopher: Ah, yeah. He's been on a Gulling spree lately.