See Art. 7: "When assessing whether consent is freely given, utmost account shall be taken of whether, inter alia, the performance of a contract, including the provision of a service, is conditional on consent to the processing of personal data that is not necessary for the performance of that contract."
See Recital 32: "Consent should be given by a clear affirmative act establishing a freely given, specific, informed and unambiguous indication of the data subject's agreement to the processing of personal data ... This could include ticking a box when visiting an internet website ... Silence, pre-ticked boxes or inactivity should not therefore constitute consent."
If you want to use external tracking and be GDPR-compliant, you must offer a clear choice ("yes/no") and you must not use pre-ticked boxes (i.e. an opt-out approach).
Please feel free to downvote this if you don't like it, but I'm merely telling you what the law says. If you disagree factually, I'd appreciate a comment though.