Later this year, when the feature is fully implemented, Workspace users with E2EE enabled will be able to send encrypted messages to any external email users. Instead of the message, recipients will receive a link that, when clicked, will take them to a restricted version of Gmail where they need to authenticate with the organization’s chosen identity provider to view the decrypted message. External users will also be able to reply within the same restricted Gmail interface.
How can it be end2end then? If you don't have some kind of public key of your recipient, but they are still able to read the message once they proofed their identity, that means that the secret to decrypt the mail is stored on Google servers and/or part of the link which is send in a non-encrypted plain-text mail.