KeeperMode: If GOG does not provide information right away why this is happening. Whether it is the developer's fault or some other.
This later leads to a heist and people spread the word that they either won't buy the game on GOG anymore, or no one informs them what's going on.
Such information spreads really fast.
Regardless of whose fault it is, GOG should clearly communicate why this is happening for its own sake, because that's what I care about.
The "why" is the same every time something like this happens: Because the dev didn't upload the game to GOG servers yet.
GOG does not monitor Steam releases. They don't have the necessary informations to communicate a reason for differences.
Some devs are very dutiful, they release on all platform (GOG, Steam, Epic, Gamejolt) on the same day. But most devs release on Steam first, on the others later.
And in sadly way too many cases some stop releasing anywhere but on Steam at some point. There are some games, where the GOG version is several years old, but the Steam version got updated repeatedly.
Check the thread about "second class citizens"