acunningham has it right.
When fetching a mark upwind, I find that the GPS isn't a lot of help. It's assuming you can travel directly to the waypoint when in reality you can't. It's far better (IMHO) to keep your eyes outside the boat.
One thing a GPS IS good for upwind is showing your track. If you notice your track 'bending' one way or the other you can get a good enough measure of drift to work out (a) the preferred tack to the mark and (b) where the lay-line is in real time - and nail the mark exactly.