Is it legal that your company can monitor you through GPS without prior notification?

In most states, there is no hard and fast rule regarding such monitoring. The fact is that eemployees typically have no expectation of privacy in the workplace, including for the most part their company vehicles. If an employee is using their own vehicle and uses it for both business and personal use, they will have a good argument that the times in which they are driving it for personal use, privacy rights apply. However, in the alternative, when they're on company time and the company is paying for the use of the vehicle, they likely do not have an expectation of privacy. As for an employee receiving prior notice of such monitoring, this can have an effect on claim of invasion of privacy. I believe this to be the case in NH.

At this point you can contact your state's department of labor to inquire further or you can consult with an employment law attorney in your area; they will best be able to advise you as to specific state law.