They're outsourcing identity verification to gmail to replace the "email verification flow". This way they don't have to configure any email infra like sendgrid, ses, etc, deal with bounces and so on. It's also possible gmail has better API support to process a lot of email, but I have seen this in products that have nothing to do with email. Just plain we only serve gmail customers. It's likely also another stricter variation of we only serve US customers.