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.
Console accounts cannot be played on other devices due to various legal restrictions that we must adhere to. 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.
If you are already playing with a different account on mobile, you can also play on Steam. 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.