Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Environmental Awareness (5 points): Kill a zombie using an environmental hazard in any game mode.
Gas Powered (5 points): Complete an entire arena, getting every kill with the Chainsaw.
Gun Shop (5 points): Kill at least one zombie with each weapon type in any game mode.
More the Merrier (10 points): Kill 10 or more zombies at the same time in any game mode.
Socialite (5 points): Play in an online multiplayer game.
Tour of the City (15 points): Complete each arena at least once in the same game in any game mode.
Can't Touch This (20 points): Rack up a 100x multiplier in any game mode.
Friends in Low Places (20 points): Save 25 survivors in any game mode.
Interior Decorator (20 points): Kill 25 zombies in any game mode using environmental hazards.
A Week To Live (25 points): Complete 7 days in any game mode without dying.
Hotter Than Hell (25 points): Complete the final day of the "7 Days of Hell" unlockable mode.
Anniversary of the dead (45 points): Complete the Final Day, in either Single or Multiplayer, and finish the game.