This is not true, to my knowledge. Heroic can download GOG cloud saves on Linux. Upload them, too. There may be edge cases for games using a launcher or their own cloud save system, and some games don't support GOG cloud saves, but you can absolutely keep using existing GOG cloud saves on Linux through Heroic, which has at least some official endorsement from CDPR and GOG (they have an affiliate link that gets them some revenue if you buy games from GOG within the Heroic launcher with ad support turned on).

I get that it's easy to miss this because GOG itself won't advertise it, you just kinda... have to know to use Heroic for this and turn the cloud saves option on. But it works. I use it on a dual boot setup and I've been playing The Alters back and forth.

For the record, I agree on some of the pains of trying to daily drive Linux for some things. Which is why I dual boot instead in the first place. And I do agree the packaging situation on Linux is absolutely bonkers and only makes sense in that ecosystem for entirely self-referential reasons that don't matter to users and shouldn't be a thing.