Here is the full list of all 11 Gun Devil achievements.
The base game contains 4 achievements, and there is 1 DLC pack containing 7 achievements.
Get the Goddaminator.
Thank for playing Gun Devil!
Defeat the idiot king.
Defeat the dumb dog.
Purchase your first weapon.
Complete the Flame River stage.
Complete the Factory stage.
Complete the Village stage.
Complete the Hay Farm stage.
Complete the Lil' Cave stage.
Complete the cavern stage.