Yesterday GoG client started some update, it says 83MB.
It goes very slow - it says 0,1 MB/s, takes forever.
But, it reached 92% and it stopped completely. Now it doesn't even say 0,1 MB/s.
What do I do now?

And anyway it takes bloody forever to apply a patch, not to mention that now it stopped.......ffs

Tell me what to do.

these are issues with GoG, and steam respectively. post on their forums, and or keep watch on their update and patches

They might be related to BG3 as well, since I don't have these issues with other games (e.g. see GOG 62G download for patch 3).

But yes, should also let the distribution platform (e.g. GOG) know, as sometimes it's on their end, such as these related 2.0.26 Beta (December 4, 2020) updates to GOG Galaxy client:

Right click on the failed download and you should have an option to resume.