When did those who don't use Gmail grant permission?
If I don't use Gmail, and don't have an account with Google, at what point did I "opt-in" to (or more likely, fail to "opt-out" of) allowing Google and others to access the content of email that I have sent to individuals who happen to use Gmail? How do I find out what permissions others have granted to Google et al. to access and use *my* data? And how do I even know for certain whether a given recipient is actually a Gmail user, given that some corporate email addresses may be Gmail in disguise, and some individuals may use Gmail to aggregate email from non-Gmail accounts?
This looks like a clear breach of the GDPR. The only real question is, who is committing an offence: Google, for allowing access to my data; third-parties for using the data for purposes for which they haven't obtained specific consent; or Gmail users, for granting Google and others access to my data without my consent? I suspect Google has the greatest liability here, for running a data processing system that fails to have GDPR-compliant mechanisms in place for safe-guarding third-party data.
Google seem to be presupposing, incorrectly, that all data associated with a particular account is the account holder's data. This is the same error in reasoning that Facebook make in their justification for shadow profiles, i.e. unlawfully holding and processing personal data relating to individuals who are not users, and refusing to protect against abuse of such data, by claiming, obtusely, that the data and the right to consent both "belong" to the account-holder who provided the data to Facebook, rather than the person whose data it is under the law.