Personal data is any information relating to an identified or identifiable person. From a drone's viewpoint that covers more than you might expect: recognisable faces, vehicle number plates, house numbers with people in frame, and even behaviour patterns — who regularly shows up where and when.
The moment footage contains personal data, GDPR governs it: the regulation decides how the material may be stored, used, shared with others and when it must be deleted. That applies to accidentally captured passers-by too, not only to deliberate filming.