On the primary subject of the thread, a case could be made that
Sins of a Solar Empire: Rebellion (from Ironclad/Stardock) belongs on the list at the moment.
As background: Steam users on the Steam non-beta channel get v1.82. The version released to GOG was v1.83, and that version was available on Steam in the beta channel around the same time as the GOG release. All good so far. The problem is that in late February, in an attempt to fix certain bugs identified in v1.83, the developer released a second build, confusingly *also* branded as v1.83, to Steam beta users. To mitigate confusion, I refer to this newer version as v1.83b, although as far as I know it is not tagged that way anywhere in the game files. Steam users can now choose between the non-beta v1.82 (which GOG never had) and the beta v1.83b (which GOG has not yet posted). GOG users have only ever had access to v1.83.
With that background, the argument could be made that GOG users get suboptimal treatment since:
1) GOG users cannot cross-play with Steam users. This worked until the Steam beta channel switched to v1.83b. Now, the GOG build is too new to play with the Steam non-beta and too old to play with the Steam beta. This would be fixed if Steam allowed users to pick their beta build, or if the developer provided v1.83b to GOG.
2) GOG users cannot benefit from whatever bug fixes are present in v1.83b. This would only be fixed if the developer provided v1.83b to GOG.
3) This kind of version skew is reasonable for a short period while the respective publishing groups get the content uploaded, links updated, etc. However, Steam's v1.83b was posted February 24 and as far as I know, there is still no word when, or if, v1.83b will be provided to GOG.
There are reports that v1.83b has not been promoted from beta to a full release due to other bugs identified while testing it. I am unclear whether those bugs affect v1.83 (offered from GOG) or are newly introduced in v1.83b while trying to fix other problems. There is an
active thread in the GOG Sins forum about the inability to cross-play, and some of the posters there are quite irate about it. Supposedly, dropping in the main executable from Steam's v1.83b (if you can get it) enables using all the other GOG content to cross-play with Steam users. Thus, if GOG had an optional download of the v1.83b executable, users willing to risk the (new?) bugs in v1.83b could get cross-play support. I have not seen word on whether GOG lacks v1.83b because GOG does not want to push more beta versions or because the developer has declined to provide the newer betas to GOG for distribution.