This is silly and dumb. GPS doesn't measure average speed. GPS measures location. Speed is calculated. It's calculated over the entire run - average speed -

This is silly and dumb. GPS doesn't measure average speed. GPS measures location. Speed is calculated. It's calculated over the entire run - average speed - and it can be calculated between any two location measurements - which can be very close to instantaneous speed. Radar guns measure location too, for that matter - distance from the gun is measured at two instances, and speed is calculated. There's absolutely no inherent reason why GPS-based speed calculations are any less reliable than a radar gun. The better GPS systems update their position 5 times per second - far more precision than is necessary for good vehicle speed readings.