I trying use GC sandbox in game, on iOS 7 it works fine, but on iOS 8 beta5 GC authentication return error "The requested operation could not be completed because this application is not recognized by Game Center". In production GC work fine on iOS 7 and 8 beta5.
You should turn on the GC sandbox mode.
go to settings --> game center
scroll to the bottom, there is a "sandbox" swith under "developer".
(Important! - These developer options are hidden until you have connected the device to XCode 6. If you didn't use XCode and deployed your app in some other way - e.g. Testflight, iPCU, iTunes, then you will not see these options)