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