I think GPS can show the general area someone is, but I don’t think it can pinpoint location or movement that precisely. Anyways, for cell phone the police can probably see what cell towers it pinged off of, but I don’t think it’s precise enough to help, since we know Martina and Zimmerman were near each other.
runners have apps which use their cell phones to track their runs and those are accurate enough to pinpoint if you are on the street or sidewalk.
you can only imagine what the police have.