The Global Positioning System is only useful for locating objects on Earth, because of the way it works. A GPS works by knowing where the satellites should be at any given time, and listening for the Doppler effect on the satellite’s signals as they pass overhead. They are designed to be used on Earth’s surface (inside the satellite’s orbit). Once you get outside of the orbit, GPS units won’t work right.
One problem is that Earth is moving also. On Earth, the satellites are always orbiting around us, and we seem to be standing still. From the point of view of an asteroid, the satellites are moving around Earth, which is itself moving around the Sun, which makes the exact location in space of the satellite much more complicated to find.
We are already tracking and predicting the orbits of hundreds of asteroids, including several near-earth asteroids. GPS units won’t tell us anything that we don’t already know. Also, imagine the cost of installing GPS units on hundreds of asteroids.