This is not outside of the realm of possible. Google does nothing to stop spammers, makes up their own non-public rules and doesn't communicate with the Internet community, so Gmail addresses are already on shaky grounds. For instance, for starters, go search through all of your email for any email sent from Google servers where the "Reply to:" doesn't match the "Mail from:", and tell me if there are any legitimate instances.
Why is this scenario a red flag in all instances? Mailing lists, including Google Groups, may have settings that set the Reply To to be different from the Mail From.
Yes. Very common. Sorry but you have a very limited view of realistic mail traffic.
Lesser evil criminals such as spammers already have a supply chain of phone numbers, residential IP addresses, etc that can be used to work around the restrictions.
Of course if you do it constantly or from third-world IP addresses, I understand they refuse your attempts.
For instance, for starters, go search through all of your email for any email sent from Google servers where the "Reply to:" doesn't match the "Mail from:", and tell me if there are any legitimate instances. We could all benefit by starting to reject all Google sourced email with a different "Reply to:".
What we really need is a blocklist / allowlist system which tracks the millions of Gmail email addresses. Has it never been seen before? Reject it with a message telling them to go sign up for the allowlist. Has it been around for a while? Allow it.
Google can't be asked to keep their own spam in check. We have to force it on them.