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Custom levels are now WORKING!
(as in you can download these from Steam workshop and then use on GoG)

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Huge thanks to Hugo from NoClip (developers) - he has instantly responded to an email and enabled the possibility to download the mods from Steam for Zombie Night Terror.
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As to how to:

1) Download the mod. You can follow the guide here

I have used the 2nd suggestion,, (easiest) and just pasted the link to a mod then clicked download.

2) Once the file is downloaded (it will come as ZIP), unpack it to the following folder, preserving any folder structure inside the archive. Replace <username> with your Windows user name.
c:\Users\<username>\AppData\LocalLow\NoClip\Zombie Night Terror\Users\Custom Levels\Play\Chapters\

Note that "Chapters" directory might not initially exist in your installation, create it if necessary.

You should now have files like CHP/CHPM and PNG inside the "Chapters" folder, and additional "Levels" subfolder,, inside that one, containing LVL/LVLM. Sample structure below, based on the "The Nuclear Facility" level.

..\Custom Levels\Play\Chapters\Big Bang Boom.chp
..\Custom Levels\Play\Chapters\Big Bang Boom.chpm
..\Custom Levels\Play\Chapters\Big Bang Boom.png
..\Custom Levels\Play\Chapters\Levels\
..\Custom Levels\Play\Chapters\Levels\The Nuclear Facility - Big Bang Boom.lvl
..\Custom Levels\Play\Chapters\Levels\The Nuclear Facility - Big Bang Boom.lvlm
..\Custom Levels\Play\Chapters\Levels\The Nuclear Facility - Opening Cinematic.lvl
..\Custom Levels\Play\Chapters\Levels\The Nuclear Facility - Opening Cinematic.lvlm
..\Custom Levels\Play\Chapters\Levels\TNF - Ending Cinematic.lvl
..\Custom Levels\Play\Chapters\Levels\TNF - Ending Cinematic.lvlm

3) Start the game, select "Community Levels" and enjoy your new levels :)