According to KnowYourMeme, "Zerg Rush is a popular online gaming term used to describe an overwhelming scale of attack carried out by one player against another in real time strategy (RTS) games. The term originates from the popular RTS game Starcraft, in which the Zerg race is notoriously known for its ability to mass-produce offensive units within a short time frame, thus allowing the player to overpower the opponent by sheer number."
When the game is over, Google lets you post your results to Google+ and shows the message "GG", a cryptic way to say "Good Game".
{ Thanks, Mati. }