What illegal behaviour? GPT-3 is only spitting out public data. Its like blaming someone taking photo of a road for the photo including some random person.
To the best of my knowledge: GDPR has no exception for public data. If you have PII, you need a reason. Consent is a reason; "needed for the requested business" (eg. home address for online shops) is a reason. "Having fun with autogenerating sentences"... doesn't seem like one.