An insult, often made jokingly and mostly on the internet. The '3000' refers to how some advertisers used to think putting '3000' or '2000' behind everything would make it cool.
<bob>: I justbought a FruitJuicer2000! :D
<pete>: stfu plz, Gaylord3000
Basically calling Halo 3 gay. The game is usually overrun by noobs and assholes in the matchmaking and has really crappy shot registration. The term is used to make fun of the game for that matter.