[dragonwriter]

> If you run a public forum and people choose to reveal things about themselves in posts, that obviously cannot be what GDPR is about.

The GDPR has explicit provisions for how covered information that is explicitly made public by its subject is treated (for instance, separate consent is not needed for processing such information); outside of those explicit rules for particular effects, though, such information is treated exactly like other personal information of the same subject matter under the GDPR.