Dear customer support.
I understand that I am not one with such a problem, but I found the only English-speaking users GOG conversation about the loss of the saving Witcher 3 after reinstalling Vindovs 10 ... I understand that it was necessary to first read about the fact that the saves are not stored in GOG, but Unfortunately, like most users, I hoped ... thought, as in Steam, for example, they will always ...
I ask you to inform us if there is at least some hope for the return saves? At least partly ... Maybe they have on your servers or in logs? GOG It gives information about the achievements obtained in the game, they can write and save ... I'm sure that you do not need to explain how it is a pity to lose everything it was passed ...

With the aid of hope and respect

Nepomes -- if you ever come back to read this -- this is a public forum, not customer support. However, I can tell you that GOG Galaxy does not, to my knowledge, support cloud saves, so if you never backed up your documents (you ALWAYS should do this) when you re-installed Windows, then your old saves are gone for good. There is nothing that GOG can do. The onus is on you to maintain your save files.