"GG" has two meanings in the world of poker.
It's short for "good game," something poker players traditionally say when they bust from an online tournament, or to someone after they have busted, complimenting their play. It's also said sarcastically, pointing out someone's flawed play or unfortunate bust-out. While the usage of "GG" comes from the online poker world, using it in the table's chat box, it has made its transition into the world of live poker as well, with people pronouncing it as "gee gee."
GG is also the name of an independent poker network, focused on the Asian poker market. GGnetwork, and one of it's most prominent skins GGpoker, have taken their name from the above-described poker term. Another well-known skin on this poker network, is Natural8.
Player 1: "wow, that was a bad beat!"
Player 2: "GG man!"
Player 1: "Thanks man, good luck in the remainder of the tournament."
Player 1: "Have you played on GG yet? They have a weekly $5k tournament!"
Player 2: "Yeah, I play on Natural8 every week, it's a really soft site!"