I previously had it set to disabled since it liked labeling itself as an essential program.

Thanks to some new update, it starts whenever I start the computer, but is somehow hidden from startup programs.

I even have the thing checkmarked to NOT start at Windows startup under Galaxy settings.

How do I fix this?

Ouch...

Never had this problem. Galaxy in my case kept it's setting for over a year now (I'm on beta branch) <- least helpful answer....

EDIT: To find out where it hides try Autoruns. You can stop from there too.

Are you running the Fall Creators Update on Windows 10 by chance? Due to that update if you restart the PC with GOG Galaxy, etc. running it will automatically start all programs that were running when the PC was turned off / restarted. The solution is to fully exit Galaxy by going to taskbar and right click -> exit or GOG COM logo -> exit, before restarting or turning off PC.

Every single time...?

It doesn't even show up in the Startup Programs.

If Galaxy is not running when you shut down / restart, not even in the background and you have disabled start up in Galaxy settings but it still starts when you PC starts then something else is going on.

However, if it is happening when you have Galaxy running in the background then most likley this is due to the fall creators update. MS has changed how this works. If a program is running when you shut down or restart the program will automatically start up again when you log back in. This is a system change that GOG will probally not try to override.

This simple solution would be to exit Galaxy when not in use before shut down or restarting... and yes that would mean everytime.