Take a careful look at the message. If it says, "The same version of the application you opened already exists on this system," and there are "Uninstall" and "Run Now" buttons, it means you already have the latest version and don't need to install/re-install.
If you're sure you need to install/re-install, you need to uninstall before that will work follow the directions under, "How do I uninstall G-Wizard?"