Here is the full list of all 12 Gauntlet achievements worth 200 gamerscore. It takes around 6-8 hours to unlock all of the achievements on Xbox 360.
During a treasure level, successfully collect 11 treasures and exit
Finish level 10 on Xbox Live with another player using default settings
Finish level 20 on Xbox Live with another player using default settings
Play Xbox Live with 4 players
Successfully kill a thief
Collect 10 treasure chests on Xbox Live using default settings
Get 1st place on in game score board with the Elf using default settings
Get 1st place on in game score board with the Warrior using default settings
Get 1st place on in game score board with the Wizard using default settings
Get 1st place on in game score board with the Valkyrie using default settings
Acquire an Xbox Live CO-OP score of 100,000 using default settings
Finish level 100 using default settings