You can try that to start fully from scratch. But before deleting, i suggest you back up the folders that i tell you to delete on what is following :
1 - uninstall fully Crusader Kings 2 and any DLC you may have for it the "normal" way , if you don't know how to, here's some guide to reach the uninstall panel on window 10 :
https://www.pcmag.com/how-to/how-to-uninstall-programs-in-windows-10 Now let's get rid of possible remnant (that could contain some corrupted files that could explain why it does not launch for you)
2 - delete the folder named " Crusader Kings II " that is located at C:\GOG Games\ (default path, if you installed it on somewhere else, go delete the "Crusader Kings II " folder where you installed it)
Then let's delete all the CK2 user configuration and possible mods (and possible corrupted files too )
3 - delete the folder "Crusader Kings II" that is located at C:\Users\YOURUSERNAME\Documents\Paradox Interactive\
with YOURUSERNAME being of course your user name on your computer
Once you have followed those 3 Steps, download (even if you have it already download it again, in case your file was corrupted too somehow) the CK2 offline installer (i don't use galaxy so can't help if still you want to use it) on your gog account page and install the game with it.
Install it on the default path ( that will be C:\GOG Games\Crusader Kings II ) to make sure there's no window 10 oddity going on.
Launch the game, it should work if you followed the 3 steps before as there should be no more corrupted files left behind that would screw up the new installation.
If it works, good, you can install now any DLC you may have for it
