whenever I try to open GOG Galaxy or start a game it shows failed to load game data and it won't let me open gog galaxy.

While GOG Galaxy isn't running,
1. Into the File Explorer address bar, paste C:\ProgramData\GOG.com\Galaxy\storage\
2. Delete the galaxy-2.0.db file. If that isn't enough, delete the entire storage folder.

There's a different workaround on page 6 of this thread that doesn't involve forcing it to rebuild the database.
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/the_failed_to_load_game_database_error

Be advised...NONE of the fixes worked for me...NONE of them at all

There's no page 6 of that thread for me, because the number of pages depends on the posts per topic page forum setting. post 101 is what you want to link to.

They have fixed the problem !
Just download the newest Galaxy client from the link on the bottom right at the front-page , then install that.
That should fix the problem. at least it did so for me. I had *NOT* started messing with the installation myself and had not attempted any of the fix suggested as I did not want to risk screwing up anything.

Thank you to everyone in forum for their fix for the problem or for trying to fix it , Thank you all !!!!

Also thank you to GOG for a speedy fix of their huge screw-up !

Official GOG post here :
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/the_failed_to_load_game_database_error/post134
and
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/the_failed_to_load_game_database_error/post137