It's possible to get MEGAQUARIUM MODS from STEAM WORKSHOP and use them in the GOG version.

To browse MODS go here:
https://steamcommunity.com/app/600480/workshop/

--- WARNING ---
THIS SOLUTION NO LONGER WORKS BECAUSE OF PROPRIETARY ISSUES,
BUT AN ALTERNATIVE SOLUTION IS EXPLAINED IN THE THIRD POST
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Unfortunately now in 2022 this site stopped working bc Steam starts to sue and seek legal actions with all sites that let you download mods from workshop claiming that every mod that is uploaded there automaticaly becomes their property :(

That's a bad news... BUT, luckily STEAM leaves the possibility to get those contents with their own tool with an anonymous login !!

I don't own the game there, and I just succefully downloaded a MOD for this game !!

The tool that STEAM gives is SteamCMD, maybe when I'll have time I'll write some better guide but here what I did:

1) Download the SteamCMD following this official guide https://developer.valvesoftware.com/wiki/SteamCMD 2) create a folder where to put all the MODS, let's say "C:\MegaquariumMODS\"

3) Run again "SteamCmd.exe", it will open a console

4) type "force_install_dir c:\MegaquariumMODS\" and press enter (the folder must be the one you created before)

5) type "login anonymous" and press enter

6) now you need 2 steam codes, one for the game and one for the MOD to download 7) so type "workshop_download_item 600480 2815755931" and press enter
everything will be downloaded in the MODS folder you set before, and it will be already extrated in a subfolder with it's ID:
"C:\MegaquariumMODS\steamapps\workshop\content\600480\2815755931"

8) type "quit" and press enter to close the console

9) now just copy the "2815755931" folder to the "%documents%\My Games\Megaquarium\Mods\" directory and start the game to see it in the list of MODS.

That's it... and actually is even better than before !!

EDIT:
Some advanced use... oneliner !!
This is an example to download the MOD explained before, but with a single command: OR as an alternative, put all your commands in a TXT and start SteamCMD with it:

The example TXT "MegaquariumMODS.txt" contains this (maybe with many MODS to download): and SteamCMD can be launched like this: EDIT2:

More advanced stuff... don't try this if you are not sure of what you are doing !!

Instead of moving around the downloaded mod folders, it's faster to make a symbolic lynk to the game "Mods" folder that will point to the SteamCMD game download folder... so that the Mods can be easily updated with SteamCMD.

First we need to rename the original Mods folder (just change "MY_USERNAME" with yours, and eventually the used drive): Example of the symlink with the paths I used before in the guide (just change "MY_USERNAME" with yours, and eventually the used drive): BE AWARE anyway that Steam has a LOT of Mods (right now 966), and if with this method you download a lot of them, the game will try to load them all at the start and could easily fill all the reasources of the PC.

So the best way to use this method is to make a SteamCMD script with like yours 20 preferred MODS (just to not exaggerate), so you can run it again when you want to keep them updated, without the need to move folders around every time.

You can now use this new open-source software on GitHub. Works better than any alternatives and has a lot of features like collection support multiple download providers and more.
https://github.com/VovoloGames/WorkshopDL

I found recently only one way available to download Steam Workshop content, this is now only possible when you
1.) have opened a Steam account
2.) purchased the game from Steam
3.) install WorkshopDL and use your Steam account credentials for your downloads in WorkshopDL (under Options - Setup your Steam Account)

so I purchased Transport Fever 2 from GoG and from Steam, and only the purchase from Steam qualifies me now to download content from the Steam Workshop using the WorkshopDL to add content to the GoG version of this game...
:.-(

Can we please have some negotiation between GoG and Steam so that we don't are forced to buy the same item twice ???

Are you sure ?

I just tried with SteamCMD anonymous login and i can download stuff for Megaquarium. (And I don't have the game on Steam.)

Be advised anyway that download rules can change by game to game, it depends on the access restrictions set by every developer.

I can confirm that this is working. I have SteamCMD installed to C:\SteamCMD because I found that when I have it in my Win10 Download folder (which is much more convenient to access for newly downloaded things) that the downloads always fail on me. However when using it from C:\SteamCMD, the downloads work 100% of the time if the Steam Workshop downloader site says that anonymous logins usually work.

This is just something to think about for anyone trying to download mods that is running into issues with download fails.