Clear Channel billboards to start tracking mobile phones in Europe

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.

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?