A person who plays video games A LOT. Usually they have no job, live with their parents, and spend the majority of the time playing it.
A lot of Game Nerds try to change the definition of the word 'game nerd' to someone who knows a lot about videos games, but they are just embarassed to embrace the fact that their only hobby is clicking buttons.
One of the biggest game nerds I know is named jose.
Person: Hey, wanna play foot ball?
Game Nerd: NOOOO, I ALMOST HAVE A SPY PLANE ON BLACK OPS.