Counting airports doesn't give you an accurate picture, because just about any airport can have a GPS approach, since the whole point is that it doesn't require any ground equipment. That doesn't mean those approaches get much use, because people don't fly into those airports all that much, especially not on instrument approaches.
I fly out of an airport with a published GPS approach and no ILS, and people use the GPS approach on well under 1% of the total landings there. Any reasonably busy airport will have ILS.
True, they are less busy, but it does not mean GPS is not used for instrument approaches (as parent implied).
I fly airplane without IFR GPS (IFR student), but ATC is trying to assign us GPS approach all the time (when coming into airport with ILS). As far as I understand, GPS approaches are used, even if they are less critical in busier airports. Besides, sometimes ILS is not available in bigger airports as well.
Neither should matter. A visual landing at any airport is pretty much the same, and the basics don't change for different aircraft. There's nothing at all tricky about a visual approach into SFO.
You've got it backwards:
CAT I is 200ft Decision Height (btw, not Minimum Descent)
Cat II is 100ft
Cat III is 0ft (with differing horizontal visibilities for IIIa/IIIb/IIIc
500 ft altitude with GPS only (without vertical guidance). LPV minimum is 300 ft.
Category III ILS minimum is 200 ft, Cat II -- 100 ft. But there are a lot of airports where ILS is not available.