Radar guns are very accurate in an ideal situation. Unfortunatley, trying to hold a radar gun in a moving boat with wind coming straight at you is less than ideal. It also matters were the gun is pointed and the srength of the reflection from the target. Another factor is if they are pointing the gun at the water, or a stationary shore target. Water current can also effect readings of both measurments.
I like to think that the GPS is more accurate in a boating situation (biased)