1st way:
Go to https://thunderstore.io/c/ultrakill/p/Maranara/Masquerade_Divinity/
Install r2modman and install plugin through mod manager

2nd way:
Go to https://thunderstore.io/c/ultrakill/p/Maranara/Masquerade_Divinity/
Install BepInEx and run ULTRAKILL once
Drag the "Masquerade Divinity" folder into ULTRAKILL/BepInEx/plugins
Launch the game

1) Go to 6-2 "Apostates of Hate" and follow the screenshots
Go to this ladder in 6-2 and climb it, then turn to the left

Here is the door you need.

There will be a book with puzzle. As you can see, we need to go to the Wrath layer and activate the secrets to pass through the locked door.

2) 5-1
You need to complete the level as normal untill you will reach the first room where Sentry enemy is introduced. There will be a crack in left wall that you can break.

You break the crack and go through the wall, untill you reach an electrical box you should break.
A message should appear "Somewhere in HERESY , a door shivers"

3) 5-2
In this level you need to go to the lighthouse and enter inside this cabin

Press this button.

4) 5-3
Complete the level as normal until you reach Gabriel's hologram.
You will see this glass panels. On a screenshot.
Break glass and get inside

Break this electrical box and you'll see this message: "Somewhere in HERESY , a door shivers"

5) Go back to 6-2 and enter the unlocked door

Congratiulations! You have unlocked a playable Gabriel!

Currently, this mod is still in development. There is only prelude and Limbo but I'd still recommend you to play it. Because this mod introduces new weapons that are completely different from V1's.

"Masquerade Divinity" by Maranara
https://thunderstore.io/c/ultrakill/p/Maranara/Masquerade_Divinity/

Amazing mod. 10/10