Apple mentions that message in Change your Apple Account primary email address - Apple Support
but its advice isn't really helpful given the other information you provide: "If you see a message that the email address you want to use isn’t available or is already in use, check that you or a family member aren’t already using that email address with a different Apple Account. If a family member is using that email address, remove it from their account, then try to use it."
Obviously Apple thinks the address has been used recently. I don't think Gmail is to the point where they are allowing people to create new accounts with previously used email addresses. So I don't know what to advise unless you are mistaken about the address not having been used in any context with Apple previously.