Unfortunately we were only able to offer the match functionality for a limit time. On top of that, it doesn't support DLC / expansions. There's no way for us to link purchases between your GOG and Steam accounts at present.
Or maybe it's not the door but something else that's bugged. The door/FoW is working fine for me.
Here it is in the editor (FoW locking highlighted - note the door itself is not highlighted):
Here it is in-game before completion:
And here it is after completion:
Was your possession...
The way it works is if a patch of locked FoW is touching a possession door, it'll be revealed when that door is destroyed. The way to use this is to put a player-owned Underminer at the end of the room, linked to one side of the door via Obsidian (if the Obsidian is destroyed, so is the door).
Fog of war locking should work independently from needing to use the Possession Door mechanics, but a Possession Door still needs to be placed somewhere on the map for you to be able to paint it.
However, without making use of the possession door there is no way to actually remove that locked...
Hey Electrolight - I've requested he add a credit to you. If he doesn't then I'll take the map down.
We don't intend to add permissions like that simply because it'll get quite messy and difficult to control.