Q: Apple ID already in use (it's not)

It is possible, I suppose, that your "new Gmail" email (is the username 'before' the @ 'common' or easily mis-typed) was used by someone else by mistake - OR on purpose

Contact Apple for help with Apple ID account security

How do you know it is not in use?

Go to iforgot.apple.com and type in your first/last name and the "new" Gmail account. If there is an Apple ID that uses all 3 items it will be displayed.

There is still a chance that someone else used that email address as there Apple ID by mistake. Creating an Apple ID does not create a new email address, so it's not like you and someone else are using the same email address.

Similarly, you could have inadvertently made a second Apple ID. This honestly happens a lot more than it should.