it didn't start at gps data from google... it went from public posts on facebook to the email and phone number account associated with that to the google account associated with that to the gps data associated with the google account. if you show me them using a reach around route to get that gps data and persecute peaceful protestors that haven't been suspected of criminal activity then i do agree it's troubling. if you want me to agree that the government is not within their rights seek evidence via normal, judge approved, subpoenas to investigate/prosecute people storming the capital and doing legitimate crimes then i disagree. you need probable cause and that bar should be fairly high.
But my point is that the article implies that there's some uncertainty as to whether this data can be used to identify everyone present at a place and time, and there isn't. It has been done before.