Beastmode

Enter the Gungeon
can be activated by going to the Options tab in the menu screen, GAMEPLAY, then checking the box next to Beastmode. The option is required to get the Beast Master Achievement and to unlock the Bait Launcher.

Beastmode has no effects on gameplay.

Exit the Gungeon
Beastmode can be activated in the Options tab in the Diginomicon. Unlike in Enter, there are multiple levels of Beastmode.

Trivia

 * The addition of this mode is likely there to mock games that use 'beast mode' as a term for a high difficulty experience.