The ePrivacy proposal describes itself as lex specialis to the GDPR. That is, it deals with a specific matter such as cookies or emails, as opposed to lex generalis, which deals with more general matters, such as personal data. Lex specialis overrides lex generalis. Therefore, the ePrivacy regulation complements and even overrides GDPR in areas where it provides more specific requirements for a given subject, such as commercial emails or access to terminal equipment.
I think I want to take a course on privacy by design.