The letters QGB stand for Queer Gay Bellend, a person who has same sex relationships and acts + looks like the tip of a penis.
It is used primarily in an insulting manner, to hurt a person and make them feel like a dickhead.
Matt you're a massive QGB!
Can't believe you just did that, you fucking QGB!
Will: "I like men"
Matt: "I knew you were a faggot QGB"