Email isn't a protocol. SMTP, POP, IMAP, JMAP are protocols.

GMail requires you to enable IMAP/POP to use it and uses proprietary auth protocol or use unsafe application passwords.

GMail IMAP also has a bunch of IMAP extensions that are unique to GMail.

Point is - primary interface to GMail is REST API that is (again) unique to GMail.

Random web apps are not. Imap pop and smtp don't sanely support mfa, so the insurance industry is slowly killing them off