A feature added into games upon the release of the Xbox 360. It was intended to be a bit of fun, but because Xbox Live is just the huge cuntfest that it is, it is now a number used to criticise other people.
If for example, someone has a low gamerscore they suck and should immediately give up on living. However, if someone has a high gamerscore then they have no life. These are not the only two parts of the spectrum though, because your gamerscore is the perfect balance of skill while still having a life.
I have a gamerscore of 77,770. Therefore everyone with 77,769 gamerscore or less sucks at playing video games. Anyone,with 77,771 gamerscore or more has no life.
A points system used on the Xbox 360. Often abbreviated as just 'G'. Every Xbox 360 game has "achievements" and when you unlock these achievements you will get a certain amount of gamerscore.
For example, one achievement on Gears of War is called "Commando" and you unlock this when you complete the game on the hardest difficulty. It is worth 30G.
Your Gamerscore is shown on your Gamercard.
"Wow look at that guy with no life - he's got 100,000G!!"