Enter the Gungeon is a fast-paced bullet hell shooter with roguelike elements, and is therefore often compared to The Binding of Isaac and Nuclear Throne. The player chooses between the four protagonists, all who have different special abilities, such as calling for support or lockpicking chests. A second player is able to join in for co-op mode and control a fifth character. While the player descends the Gungeon, they are set to go through multiple floors, each with a random number of rooms in it. While the rooms are pre-defined, the constellation of rooms in a floor is procedurally generated. Each room contains a set of enemies, which vary in strength, endurance, and attack behaviour, where the attack can range from simple, straightforward shots to a complicated mixture of shots fired at the same time. The player may dodge attacks by performing a dodge roll, which was inspired by the Souls series of video games, and are invulnerable during the action, or alternatively flip tables and use them as covers, although tables can be destroyed if they are shot at. The player has a limited number of "blanks" for each floor that can be used to eliminate all current projectiles and temporarily stun enemies. To defeat enemies, the player must use guns, which can be found in chests, won by defeating bosses, or bought at shops scattered around the floors in the Gungeon. The game features over 200 different guns and items that can be combined to achieve more powerful effects. At the end of each floor, a boss awaits the player; beating the boss grants the player currency to spend at shops and unlocks the next floor.
As the player progresses through multiple playthroughs, they may encounter non-player characters that can be rescued from the Gungeon. Once rescued, these characters take residence at the Breach, a safe level above the Gungeon, and where the player, prior to starting a new playthrough, can spend a type of in-game currency earned from boss fights to permanently unlock special items that will then have a chance of appearing within the Gungeon for all subsequent playthroughs.
Enter the Gungeon is set on a distant planet inhabited by living bullets and other strange lifeforms. A gun that could kill the past itself was kept deep within a grim fortress, until a deadly force tore the fortress down and reduced it to ashes. The fortress was rebuilt with the highest of security measures and four adventurers, each with their own stories and regrets, decide to enter the fortress and descend into the Gungeon to seek the legendary gun in order to defeat their own past.
Development on Enter the Gungeon started in 2014, with four Mythic Entertainment employees leaving the company to fullfil their own project just before the company would shut down later that year. According to developer Dave Crooks, he had been listening to the soundtrack to the game Gun Godz by Vlambeer, and the name "gungeon" came to him the next day. Crooks presented the name Enter the Gungeon to his fellow team members, and they fleshed out the game's lore over a lunch meeting and then spent the next few weeks to prototype the game mechanics. Though Crooks stated that The Binding of Isaac was one of the game's biggest influences, they also were influenced by Nuclear Throne, Spelunky, Dark Souls and Metal Gear Solid.
Dungeons in the game are generated in a procedural manner, but they found it was better to handcraft the individual rooms, playtesting those individually, and then using their random generation to connect these rooms into a dungeon. The designs of the guns took place over the two years of development, with most of the designs by team artist Joe Harty; several of the guns are inspired by other video games and video game systems, including the NES Zapper and guns similar to those appearing in games such as Mega Man, Metroid, Shadow Warrior and Serious Sam. The boss character designs were made by a combination of ideas from Crooks and Harty, which then fed into the gameplay programmer David Rubel to determine appropriate bullet hell patterns associated with that idea.
The dodge roll mechanic was inspired by trying to include a similar mechanic of Ikaruga that enabled a player to easily dodge numerous bullets simultaneously, and took the ideas used in the Dark Souls series to have the character dodge out of the way. The team loved this mechanic so much that they opted to name their studio after it. Similarly, they included usable environmental features such as flipping tables or bringing chandeliers down onto enemies to encourage the player to interact with and use the environment to their own advantage. At one point they had included an active reload feature, similar to Gears of War in which pressing a controller button at the right time during a reload would increase the damage the reloaded bullets would do, but instead decided to limit this to a power-up that can be collected, finding that players were already distracted enough by everything else going on in the game and that felt the moment of tension when the player had to wait for the gun to reload was critical to gameplay.
Dodge Roll eventually signed a contract with Devolver Digital. In December 2014, at PlayStation Experience, the game was officially announced and followed by an announcement trailer. Throughout 2015, the game went to multiple conventions, including E3 2015, where, during the PC Gaming Show, the co-op feature of the game was revealed. On March 2, 2016, it was announced that the game will release on April 5, 2016. The game is set to receive free post-release content, such as more weapons, enemies, and levels. The first of these, the "Supply Drop" update, which adds several new guns, enemies, and room layouts, was released for free on January 26, 2017. The developers anticipate releasing a major update sometime in 2017 or 2018.
Upon release, Enter the Gungeon received mostly positive reviews, and holds a Metacritic score of 84/100 for PC and one of 82/100 for PlayStation 4. Jed Whitaker of Destructoid rated the game 9.5/10, with praise that the repetitive entering of Gungeon "never quite feels all that repetitious", however stating that "[t]he only part of Gungeon that will surely turn off some people is the difficulty." Electronic Gaming Monthly's Spencer Campbell summarizes the game as "overwhelming", giving it a score of 9.0/10, however, also decries the difficulty to be too high by saying "Even though there were a few times that I yearned  for an easier difficulty [...], Enter the Gungeon's difficulty is actually one of the game's strong points." Vince Ingenito from IGN regards the large variety of guns in the game to be especially pleasing, but stating that a few guns are not very original or plain boring, rating the game 8.5/10. For Hardcore Gamer, Kyle LeClair gave Enter the Gungeon a score of 4.5/5, highlighting the game's "terrific sense of humor, astonishing action, and style and charm oozing out of every corner of its pixelated world", naming it to be "[a]n absolute blast at every turn and just pure concentrated fun overall". The Escapist's Joshua Vanderwall gave the game 3.5/5 stars, criticizing that "the difficulty often comes from the game refusing to provide you the necessary tools", also stating that "[t]he learning curve is very reasonable at first, but there are regular spikes that less proficient players will find incredibly frustrating".
Enter the Gungeon sold more than 200,000 copies within its first week across all platforms, with Steam Spy suggesting that about 75% of these sales were made on Steam. By January 2017, the game has sold more than 800,000 units on all platforms, according to Devolver Digital.