If the GameStop app isn't allowing it, that's something GameStop would need to address. However even if you did get it installed, you may not be able to run the game as the registration system does not support the Win8 betas.
If you have GC2 Ultimate, you can just download the game straight from the Stardock store. Otherwise, if GameStop can't change the OS install permissions, support@stardock.com may be able to help get the game by other means and/or provide a registration workaround.