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.
Defeat the dumb dog.
Defeat the idiot king.
Get the Goddaminator.
Thank for playing Gun Devil!
Complete the cavern stage.
Complete the Lil' Cave stage.
Complete the Hay Farm stage.
Complete the Village stage.
Complete the Factory stage.
Complete the Flame River stage.
Purchase your first weapon.