Apps should sign users into their GameCircle account across all devices that use the same Amazon account. If users have not setup their GameCircle account then they will be signed in as guests. In the event that a user does have a GameCircle account but is unable to log in, there are a few things to check.
The application could be looking for Google Play Services to initialize GetDeviceDetails(). A ClassNotFound exception will be thrown since Google Play Services do not work on Amazon devices, which will result in a "No Init method found on extension class:GetDeviceDetails returned Init " error message. GameCircle relies on device information from Amazon platforms to detect GameCircle players. Without this information users will be recognized as "GUEST".
Attempting to integrating GameCircle via third party tools also cause problems. GameCirle only fully supports the specific platforms.
If errors still occur, please sign into the Developer Console and Contact Us.
More information on GameCircle can be found in the developer documentation: