Slime Rancher just got a cosmetics DLC on Steam that isn't on GOG, called the Secret Style Pack. Interesting to note that this also is on the Epic Games Store (and not the other DLC). I tried asking on the Steam forums whether there are any plans to bring the DLC to GOG but didn't get any answers.

While it's possible that a GOG release of this particular DLC is planned (Grim Dawn also just got a cosmetics DLC pack that isn't on GOG yet, but might since GOG has all DLC so far), considering the total lack of Slime Rancher DLC here, I'm not holding my breath.

When asked about the free Galactic Bundle DLC, Monomi Park mentioned that they do not currently have a way to make their DLC work with a DRM free build of the game. I presume this applies to the Secret Style Pack as well, which would mean chances of seeing it here are slim to none. :/

Yup, we're getting screwed over again:
"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

Suddenly I see why GOG Galaxy 2.0 will support launching other games and other clients.

Tempest has released Pirate City DLC May 29th 2019 on Steam. No response about a GoG release in the forum.

If I read such things I would love to contact the devs and kindly ask for X% refund to compensate for my loss. I definitely payed too much.