Full list of all 54 Enter the Gungeon achievements worth 1,250 gamerscore. It takes around 80-100 hours to unlock all of the achievements in the base game.
The base game contains 51 achievements worth 1,000 Gamerscore, and there is 1 DLC pack containing 3 achievements worth 250 Gamerscore.
Kill the Bullet's Past
Unlock the Bullet
Map the first Five Chambers for the lost adventurer
Complete Daisuke's trial
Populate the Breach
Kill a Jammed Boss
Access the Temple
Complete the game with the Sorceress's Enchanted Gun
Clear the Fifth Chamber 10 Times
Clear the Fourth Chamber 20 Times
Ace Winchester's game 3 times
Open the shortcut to the Fifth Chamber
Accessed the Hidden Market
Clear the Third Chamber 30 Times
Kill the Robot's Past
Complete the game with Beast Mode on
Open the shortcut to the Fourth Chamber
Clear the Sixth Chamber
Collect five Master Rounds in one run
Kill 100 enemies by dropping chandeliers
Unlock the Robot
Complete all 4 main character Pasts
Kill the Hunter's Past
Kill the Convict's Past
Steal 10 things
Deliver the Golem's replacement arm
Kill the Marine's Past
Die in the Past
Open the shortcut to the Third Chamber
Kill the Pilot's Past
Fall at the last second
Construct the Bullet that can kill the Past
You've met with a terrible fate, haven't you
Kill 100 enemies while riding in a mine cart
Clear the Second Chamber 40 Times
Carry a large amount of money at once
Clear the First Chamber 50 Times
Kill 100 enemies by knocking them into pits
Get the jump on a Mimic
Kill a frozen enemy by rolling into it
Open the shortcut to the Second Chamber
Why
Defeat the Boss of the Fifth Chamber
Win a wager against the Gunsling King 5 times
Spend big at the Acquisitions Department
Always be flipping. Guns are for flippers
Avenge Manuel
Kill a boss after covering it with glitter
Access the Oubliette
Complete the Tutorial
Complete Frifle's Challenges
Defeat an Advanced Boss
Complete Tonic's Challenge
Take Revenge