Here is the full list of all 12 Counter-Strike: Global Offensive achievements worth 400 gamerscore.
Play 5,000 matches of Arms Race or Demolition mode
Kill 10,000 Enemies
Earn 100 awards
Personally kill the entire Terrorist team before the bomb is planted in Online Demolition Mode
Kill the entire opposing team without any members of your team taking any damage
Get a kill with every weapon
Kill 250 enemies with headshots
Win an Arms Race match without dying
Dominate an enemy
Kill an enemy with the last bullet in your magazine (excluding sniper rifles and Zeus x27)
Earn $50,000 total cash
Inflict 2,500 total points of damage to enemies