Self driving encompasses a bunch of things. Obstacle detection is one. Vehicle control, including micro adjustments, is one. Path planning is another one.
GPS is one way of doing one part of self driving. But it's ALL self driving
A pre-planned route != self-driving (in my opinion).
I say that because it's not driving itself, it's driving a route you plotted or recorded.
You can't take that same tractor and have it work on your neighbors pasture as-is, it has to be programmed.
But again, this is getting into a semantics debate. Personally I'd prefer to differentiate using autonomous and automated, self-driving is too ambiguous.
By that logic, am I driving the car while following the route plotted and dictated to me by Google Maps? What if it's all over two-lane freeways, so all I have to do is stay on track, not crash into the vehicle in front of me, and follow Google's instructions? And what if I then turn on adaptive cruise control? Am I still driving the car?
GPS is one way of doing one part of self driving. But it's ALL self driving