So here is the thing, I quit witcher 3 yesterday, nothing bad happened.
But when I returned today, the 16 DLCs appearently unchecked themselves AND deleted themselves, so now I have to download them again....

What's up with that?

I can't help but you should disable auto updates, last days I got two updates that ended do nothing apparent, but at least it wasn't sneaky updates, I applied them manually.

My personal experience is sneaky updates is a pure bad design and that's a terrible weakness of Steam (need be offline to not be forced to apply last update) so on Gog take profit of this extra power at your disposal, check no sneaky updates.