For an iOS device, you need to first logout of Facebook in BattleGems, then you need to go to your system settings on your iOS device, go to Facebook, the toggle on/off for BattleGems, then login to Facebook again via BattleGems.
For an Android device, you should logout of Facebook in BattleGems, remove or toggle permission for BattleGems in Facebook via your web browser, click on Apps, find BattleGems and then click on the settings gear to reconnect to Facebook.