Just bought this game, but I noticed it has steam workshop support on the Steam version

Is there any way to download and play levels made by other users on the GOG version?

Have you tried http://steamworkshopdownloader.com ?

Gonna apologize for the massive necro first off.

I must say that the game has downloads via steamworkshopdownloader forbidden, so that does not work.
However, 2 of the 36 workshop items are also on the game's NexusMods:
https://www.nexusmods.com/butcher

Past that, you are flat out of luck unless you don't mind buying the Steam version. Being only $10, I'd say maybe if the custom level scene/community were still active. But as it stands now, it's dead like a well burnt steak.