What a disappointment GOG Galaxy is.
First I could not install it.
Finally its working.
Downloaded Fallout 4 GOTY, it failed to update.
Manually verfying the files, it detects two issues: 1) cannot sync saves, 2) cannot update.
So after clicking repair it finds an update. OK It is successfully finished updating the game this time.
I double check and click repair again, and it found yet again new update.

Fallout London is a long big mod for Fallout 4.
How the hell can I be sure that everything is installed properly with such poor behaviour from GOG Galaxy?

Download and manually install the offline installers instead.

yea