[dragonwriter]

> IANAL but GDPR refers to personal data collected from users.

No, it addresses the collection and processing of personal data no matter where it is collected from.

> The only "Youtuber data" being "used" here is publicly gettable data from the Youtuber's channel.

Which is personal data that YouTube has collected or created associated with services they are providing to the user. They are responsible for their collection and use of it, but that doesn't authorize third-party collection and use of it under the GDPR. Particularly, the GDPR definition of personal data subject to the rules it imposes does not exclude information which is “publicly gettable” from some other party than the subject.