Since GOG users can't browse the online collection from Steam and Steam users can't play levels from GOG players, I have created an online level library where everyone can upload & download levels.

Read all the instructions here:
https://docs.google.com/document/d/1TaVFc2IHIlGS63Zo1xkx0OtHE4ZqbrBblaVA3RsdZn4

Public Levels Library:
https://drive.google.com/drive/u/3/folders/1Ypo-cQWkIYHsHiS70XZ_kCdhLzGTj9SD

Upload location:
https://drive.google.com/drive/u/3/folders/1JYrYkMIe3pT712FkaZEZUkNeCm1TewmB

Please note that every user is responsible for their own content. I don't hold the right to upload other users' levels and you should also upload only levels you have the rights for.

Happy playing & sharing! :)