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