Here is the full list of all 21 Guns, Gore & Cannoli achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Complete the game on normal mode
Complete the game on hard mode
Complete the game on impossible mode
Complete the game on easy mode
Survive The construction site without killing any enemies with weapons.
Survive the sewer using only your Lupara
Defeat Mickey
Survive the river level using only your magnum
Kill 2000 enemies
Survive the army stronghold without killing any enemies.
Complete 'Bonnino's Stripclub' without dying once on Normal Mode
Defeat Sonny
Destroy 10 cars
Kill 20 enemies in the alleystreets, during one run, by kicking them into fire.
Consecutively kill 25 enemies with a headshot
Kill 1000 enemies
10 multi-enemy grenade kills
Eat 10 Cannoli
Kill 400 enemies
50 headshots
5 multi-enemy grenade kills