That's an absolutely ridiculous idea and a good way to destroy the amount of users you're getting. Do you whitelist Gmail? Then it's piss simple to sign up to multiple accounts. Do you not? Good luck a massive portion of the internet to sign up...
Gmail itself is allowed, but the addresses are normalized. I'm well aware of its potential for abuse.
I personally own over 100 from a couple years back when recapta was easily automateable and the phone-number requirement wasn't there.
I have an email address for every site I sign up for.
Helps you figure out who sells your email.
How do you handle this? Or do you not?
I guess you could do a DNS lookup of the mail exchange records and see if it points back to gmail and compile a list of domains to allow one account from... but then that would break many companies emails. That’s no fun.
Services need a better way to figure out fraudulent or abusive behaviour than guessing based on the email account's domain name.