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. :/

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.

It has just been released on GOG:
https://www.gog.com/forum/general/release_tempest_pirate_city_7c507

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Added
Aarklash: Legacy, missing Artbook
Downfall: Redux, missing OST
Hand of Fate 2, missing OST
My Friend Pedro, missing OST
Vampyr, missing The Hunters Heirlooms DLC

Updated
RIOT - Civil Unrest, missing OST and Mac+Linux build
Slime Rancher, missing Secret Style Pack DLC

Removed Punch Club, should be up to date now.

Contacted tinyBuild, check out this thread to find out what happened.
https://www.gog.com/forum/general/lets_talk_about_tinybuild_and_gog

I don't know if it is exact, but according to KillerBee69's review (from 2013) about Divinity dragon commander:

"Multiplayer is nice. You can play in teams. Just a quick skirmish or strategy maps.
BUT the multiplayer lobby is steam-exclusive. Vault and GOG versions can only play via LAN or DirectIPConnect. Larian is still writing "GOG, Steam, Vault etc... No differences" on their HP so they mislead at least me quite bad. They provide NO official substitute platform for non-steam users, not even a forum. MP is non-existent outside steam and since SP is shortlifed on its own."

Neither Larian or GOG has the ability to host a server like Steam and that's the reality of it, Lan and DirectIPConnect is the substitute even if it's not official or what people want. Unfortunate but not second treatment.

I guess GOG could possible negotiate a deal with Steam to allow Galaxy to use Steam for MP but then we run into other issues, the game was never big anyway so finding mp matches couldn't have been easy some time after release so in that case the other mp solutions make sense even if they're limited.

Update:
Hello Neighbor
Missing "Huge Patch 1.3" since November 2018.

Tooth and Tail has still got no Linux build that they've got on Steam.