You'll need your hidden Apple email address to sign in to https://www.playgwent.com/
Please follow the steps below:
1. Settings
2. Password & Security
3. Apple ID logins
4. GWENT: The Witcher Card Game
5. Your Apple email is visible under HIDE MY EMAIL
For more information on how to manage apps you use with Apple Sign in, please visit https://support.apple.com/en-us/HT210426#change