There is a device called BPCS in 3 phaser locomotives which doesn't allow train speed to go up above a barrier, in case of Gatimaan express it is set at 160km/h at max.
GPS reading varies with hardware, elevation changes, signal levels from respective sats etc, it shouldn't be a reliable source for top speed of trains, try running tests with your own vehicle, the 2 things speedometer on your vehicle or locomotive and GPS work different ways and Speedometer is more trusted since we have sensors right on the vehicle which calculate through rotation speed, the speedometer can be wrong in case of wheel slips only.