Google would need to know that remus@gmail.com belonged to the person making the GDPR request, or it would be an attack vector. From the providers point of view

Google would need to know that remus@gmail.com belonged to the person making the GDPR request, or it would be an attack vector. From the providers point of view, you are asking for a copy of your data and data about you, and they are not going to give out data that might be yours. Maybe if you had linked a phone number, but even that is arguable.