A state of mind or demeanor that one has when they are ready to party. If you are ready to party, you have your gameface plastered on, and if you aren't you have it stashed in your pocket. A group of people who are always ready to party might be nicknamed the "gameface girls" or "gameface guys"
If you call a friend and ask them to go out, you might say.."is your gameface on tonight, or is it in the pocket?" And they would reply "my gameface is plastered! come pick me up!"