Mobile devices are location aware to one degree or another. Unfortunately the term GPS seems to be used more than it should so I thought it might make sense to define the technologies in a bit of detail in an effort to dispel confusion.
One of the quickest yet least accurate methods of determining location is to use cell towers because their location is fixed and known. By looking at the relative strength of a device's signal, you can work out roughly how far a device is from a particular cell tower. You can conclude a device is within some radius of a cell tower.