Here's how I resolved the game center issue:
I returned my iPad Mini WiFi to iOS 8.4.1 hoping that I might be able to play my games again. Game Center in iOS 8.4.1 works fine but some of the installed apps in the iPad was not recognized so I update again to iOS 9.0.1 but i logged out of game center first before i update the OS. After the update all the apps are recognized and i can play all my games again. I never log in the Game Center again until now. You may try it and i hope it works for you too.
Now i have a question..... Since i logged out of game center and playing my games how will logging back in the Game Center (once Apple officially fixed it) affect my games progress? Will my games progress revert back or will just continue?