GPS tracking is certainly a possibility, which does raise quite a few privacy concerns. However, we do have to consider that almost all of already carry devices that track our location every second of every day. That being said, there is important distinction between devices that MIGHT be used to track our locations, as opposed to a device that is required by law to track your location.
Just because GPS tracking is a possibility, doesn't mean it's the only option. In fact, I would say that GPS tracking is the least likely option. A must more sensible option would be an annual tax based on average mileage which is charged to the owners of electric cars. For example, the current federal gas tax is $0.184 per gallon, and the average american driver will drive 13,500 miles per year. Also, the 2017 Tesla Model S P100D gets 92MPGe. Based on this, the federal government could charge a $26 per year electric vehicle tax to the owners of the 2017 P100D.
It's a simple way to assess a tax to the drivers of electric vehicles for their equivalent fuel usage. However, it's possible that the electric vehicle tax credit would also have to be repealed to keep from offsetting this type of gasoline equivalency tax.