Hmm, I have never had to do anything special. I just load a "sailplan" and turn on the autopilot. Making sure I am in NAV mode, with the toggle switch in GPS, and the water rudders down. I get up some speed, well, full throttle and it always tracks. It isn't exactly like in a plane. It needs time to pull itself over to the line, and when rounding a corner will overshoot and come back.
On the GPS unit, pressing the flightplan (FPL) button switches between the list of waypoints and map view. I have been working on trying to make the turning more firm, and quicker. But the version you have should track just fine. If you have your controls set up for dual throttle, you can use the engines to help steer you around a corner. Give more throttle to the outside of the corner. for example, when turning right, give full left engine and cut power to the right one. Don't forget to balance the power again once you are out of the turn.
The Grenville SP, I think, starts out with a sharp right turn, so you won't see the "line" until you turn to the right and start down the channel. If you zoom in with the GPS, you'll be able to see where it's going better.