In my app I use Google Sign In and Game Center Sign In. My app rejected due to Guideline 4.8 - Design - Sign in with Apple. I already have Game Center sign in so do i still have to implement Apple Sign In? I asked to apple team about it they didn't give a proper answer.
Yes, it is mandatory to implement Apple Sign-In because you have implemented a social sign-in (Google).
The only scenarios when you are allowed to skip Apple Sign-In, while providing a social sign-in are mentioned under guideline 4.8 (link for reference).
Game-Center sign-in is not equivalent to Apple Sign-In and cannot take its place.