Witcher 2 says "updated" in my library but I don't see anything new. and the version is still 3.4.0
am I missing something?

I believe that a Linux version has been uploaded and that has set the 'update' flag to on. If you're running the windows/mac version then nothing has been updated for those so no need to download anything. I do wish that GOG would actually clearly flag what has been updated on the game page but they never do :( Anyway, hope this helps.

Oh thanks for that ... I was afraid I will have to download 15GB again.... Thanks for your post!

There is usually a sticky thread created in the forums stating what has been updated. Not the best system, but it does let you check what the update is before re-downloading anything.

thank you