They are completely different tanks. The Panther II was a project begun before the development of the G. The major impetus behind it's design was to develop common components between the Panther and the Tiger II, as well as to improve the armor protection of the Panther (thicker armor pretty much all around and re-designed turret). As far as the status of development, a couple of hulls were completed (one sits 40 miles south of me at the Patton Museum) but no turrets, and then the project was cancelled. Some of the more subtle design changes were incorporated in the G, I believe. The Panther F, on the other hand, was simply intended to be the next ausf. of the Panther. The hull was to have only minor revisions (mostly concerning the roof plate/hatches above the drivers' compartment; Some of these hulls were being produced at wars' end), but the turret was to be completely re-designed. It's appearance was very similar to the turret on the drawing board for the Panther II, but (according to Jentz) they were not the same turrets. Some questions remain as to how far along turret development was, but I think the general consensus is that at least one prototype was finished (wound up on a British firing range after the war).
Hope this helps!