Professional MFA Google authenticator

Now we are considering to adopt Google authenticator as professional MFA.
We operate B to B Saas named GC.
We offer clients GC, and clients register end-users to their accounts.
When end-users log-in GC, they must input registered e-mail address and password now.
If we adopt Google authenticator as professional MFA,
• Must registered e-mail address be google account?
• Can GC select to allow clients MFA or not?
• Can clients select to apply users MFA or not?