Gps speed is your speed of travel over the surface of the earth. Speed on the paddle (paddle wheel on speed capable transducers)refers to the the speed at which the water is passing under your boat. Makes a big difference on the action of your gear. Thats why most people prefer to use the angle of the downrigger wires to guage the action of their gear. the only drawback with using just the wire angle is that if youre trolling into a strong current trying to keep a certain angle there are times when you could be moving backwards or as cigar pointed out sitting stationary(plays hell on catch ratio)