Here is the full list of all 12 The Gauntlet achievements.
Complete the first gauntlet in campaign mode.
Complete the campaign mode in first place.
Earn 3 Stars in Gauntlet 1 Speed Run Mode.
Earn 3 Stars in Gauntlet 2 Speed Run Mode.
Earn 3 Stars in Gauntlet 3 Speed Run Mode.
Earn 3 Stars in Gauntlet 4 Speed Run Mode.
Earn 3 Stars in Gauntlet 5 Speed Run Mode.
Earn 3 Stars in all Gauntlets in Speed Run Mode.
Attempt a Gauntlet 100 times.
Fall into the acid in Gauntlet 2.
Do not leave the end elevator for 60 seconds.