Here is the full list of all 20 Six-Guns (Windows) achievements worth 200 gamerscore.
Open 200 chests.
Use all the Road Signs in the game.
Win 25 matches in Multiplayer mode.
Reach level 25.
Kill a Giant Werewolf.
Kill one demon of each type.
Obtain 50,000 coins.
Kill 1,000 enemies.
Kill 500 enemies.
Reach level 15.
Die once.
Visit Oregon.
Kill your own horse.
Try your luck with the Lottery
Complete the tutorial mission.
Kill one animal of each kind.
Kill 10,000 enemies
Reach level 40.
Reach level 50.
Complete all quests from beginning to end.