So, I came back home after a week and GOG is downloading a 63GB update for the game.
Is it normal? Is the new patch+hotfix THAT big?

I have Steam and it was single digit for me. Can't remember exactly what size, but the downloading part was done in about half an hour.

I know some services will say "updating 99GB" (or whatever size the game is) for any size patch, but I don't know if GOG falls in that category.

It says 14% compleated at 7.7/63GB where 14% of 63GB should be around 9GB.
Idk, I only hope it isn't really downloading again the whole game for no reason.

I think it was about 3 gb

It was only 3 GB for me. Perhaps you didn't download a patch prior to Hotfix #4?