personthingy: Sorry for the double post but there was a response from the devs:
"While GOG Galaxy receives all of our free updates, our GOG builds do not currently support external DLC. We will definitely keep you in mind if we look to expand the scope of support." https://twitter.com/monomipark/status/1142176229782515712 So GOG gets shafted,
again...
It's been around 5 years since then, so one would think the company would have found a way for the DLCs to work. Then again, they are working on Slime Rancher 2 that's in early access, so they must be putting everyone on that game. Maybe after the full game comes out, they'll work on the GOG friendly DLCs for this game. It's wishful thinking I know, but you never know.
mishabrun: "our GOG builds don't support external DLC".
What makes this game different to ANY OTHER GAME that has external DLC here? Let's look at Stellaris as a prime example.
Even Dredge has all three of its DLCs on GOG, and that game came out last year. Though, I'm not a programmer, so I don't know how difficult this topic is.