1) Make sure your device is supported. Here’s some info from the official support website:
https://support.cdprojektred.com/en/gwent/ios/gwent-technical/issue/1231/supported-devices
You can also try reinstalling the game. Sometimes that helps.
2) The Gwent settings let you mute the audio. After doing so you should be able play background music while playing Gwent. With the standard music apps from apple that are installed on ios devices that should work. Or did you mean something else?
3) You need to be more specific, otherwise it‘s impossible to reproduce the problem.