Due to legal and platform restrictions, it's not possible to link a Gems of War account to or from Xbox.
If you can't see your password or an "Account" option in Settings, this is because Gems of War does not support account linking between consoles and other devices. However, your account progress on Xbox and PS4 is tied to your Xbox Live and PSN accounts respectively, so you can still reinstall the game when logged into these without a loss of progress.
Linking between mobile and PC
You can play the same Gems of War game account on mobile and PC. However, you'll have to start a new game if you are playing on console and want to play on Steam or mobile as well.
TIP: Playing on Steam/mobile? Check out this article to link your account between devices.