Clear Channel billboards to start tracking mobile phones in Europe (theverge.com)

I can't imagine how this is going to work with the GDPR in place? How should someone give their consent to this data collection and how do you remove it again?

Earlier discussion when this was rolled out in the US 4 years ago: https://news.ycombinator.com/item?id=11196671

It seems like they are combining location data sold to them by mobile carriers with limited GPS data that they might be cobbling together from several sources.