One thing to keep in mind about a GPS VMG reading...as you progress along the course towards the laylines, the angle to the mark changes considerably, so the GPS unit is constantly re-calculating the VMG. Once you get the mark off your beam, VMG goes to zero and then negative as you sail beyond 90 deg. This can be handy if you know you can tack through 90 degrees (or something close to 90) as a way of "seeing" the layline.

To more accurately train for better performance, you'd do better to set the GPS waypoint very far upwind/downwind (+50 miles?) beyond the mark you are really heading at...so the angle changes are much less significant.