I've been away from any gaming for 9 months, and now as I have returned wanted to play Grim Dawn. Starting GOG Galaxy required an update of the game, but it stuck and then said that my copy of the game is damaged, try agan or uninatall. I uninstalled it, reinstalled Galaxy, and now I can't install Grim Dawn at all - it stays stuck on initialisation screen until I cancel it! Whats going on? I had zero problems with GD before!

No idea, but I'd contact GOG support to see if they have any ideas.

Well, you should do two things.
1 Delete GD folder from disc.
2 Uninstall GOG Galaxy.
After that you should reinstall GOG Galaxy, then install Grim Dawn.
Or you can even try to just reinstall Galaxy. I found it may be corrupted, sometimes. At least it worked for me several times.