Q: How Do You Verify a Gmail Account?
A: If you mean how does one verify that a gmail account belongs to who it claims… no method is available that is reasonably accurate.
There are options to vet ID. If the sender has been assigned a class 3 S/MIME certificate, a background check is done to issue the certificate. In this case, you have reasonable assurance the person behind the email is who they claim to be.
But exceptionally few people do this.
You can use encryption, such as 7zip, and provide the password to the person in person, so you know the right person has the password. The two of you can then exchange encrypted email that only they can read.