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