Here is the full list of all 12 Gatling Gears achievements.
Complete an area in co-op
Learn how to operate your walker and complete Bootcamp
Complete all areas on the hardest difficulty
Defeat Julius Steelwell and save the world by destroying The Butler
Don't push over any trees in Trespassers
Clear all Survival mode arenas
Defeat General Brutus by destroying The Excavator
Complete a Survival mode arena without losing a life
Don't take any damage from sea mines in Tide of Battle
Destroy all lightning harvesters in Fog of War
Fully upgrade your walker
Complete an area in Katharsis without getting hit