All info and DL here:
https://www.doomworld.com/forum/topic/149566-below-the-gaudy-goblin/

Zan himself has given it a highly positive plug on Steam, works just like any other Doom WAD but with Hedon.

...So it's just a Doom wad?

Do you need to play it within the campaign or can it it be played individually?

It has its own option in the main menu screen. When finished it jumps to the next official level as the modder set it between two in the main story. There's an included read me explaining the setting in the main game.

I see, thanks!