Brave Frontier holds gems in separate wallets for the various major platforms - Google, iTunes and Amazon. If you switch between platforms (switching from an Android phone to an iPhone, for example), you will not be able to access the gems you have earned on your android device when playing your account on an iOS device. If you earn gems on the iOS device, you will not be able to combine or access those gems when you return to your android device.
You can always return to your previous platform to access the gems that you left in that platform.
If you want to know more about the term platform, kindly refer to the link Here.