idbeholdME: Serious Sam 4 can be added to the list. In the past, it was missing seasonal events (Halloween, Christmas) but most recently, since the latest 1.08 patch, the GOG version is missing the unrestricted .exe, which is required to play the game with mods. The mods (and tools for them) are also available only on Steam, but the mods themselves can be downloaded externally and they could be used in the GOG version, if Croteam provided us at least with the unrestricted .exe, if not the mod tools themselves.
There is a thread on the Steam forums, but it's unfortunately not seeing that much traffic:
https://steamcommunity.com/app/257420/discussions/0/3165442751833267189/
A restricted executable on GOG, but not on Steam. Have I entered an alternative universe or something? Even the pirated versions of the game support mods.