Why are you using double equals in this situation? When you write:
GridGenerator_Script.ValidGridSize == true
this simply returns a True or a False value, and you seem to do nothing with it...
To affect a value, you would simply use the equal sign (=).
GridGenerator_Script.ValidGridSize = true
But anyway, once you loaded your script, it needs to be instantiated in order to use it dynamically. When you use the load function, the object is not the script itself... It's a packed class. So skysphr is right, you need to instantiate it with the .new() method.
Then, do not forget to change your double equals for equals, and everything should work!
PS: If you didn't want to instantiate the script in order to use it like a global script, you should really read about Singletons. This way, the script is instantiated once (for all the game) and you still can play with its values.