Locating mobiles/UE by base stations in a cellular system
GPS is outside of operator's control. 
GPS signal is much weaker than cellular signal. You can get phone coverage in a concrete building (via windows) where you may not have the ability to receive GPS.
Geographical position does not have to correspond to the physical signal path (unless in vacuum). Imagine that you are close to one base station in front of you. A big, metal bus appears in front of you and suddenly the "better" path is via a reflection to a base station behind you (and a handover occurs), yet you did not even move a meter.