Here is the full list of all 15 GunHero achievements.
Complete the desert area
Kill 500 enemies with the SMG
Complete the winter forest area
Complete the jungle area
Complete the prison area
Complete the final area
Destroy 10000 boxes
Kill 500 enemies with the LMG
Kill 500 enemies with the uzi
Kill 500 enemies with the rifle
Kill 100 enemies with grenades
Complete a level with multiple players
Release all the prisoners in the game
Complete the canyon area
Create and publish a custom level