I am assuming that any region guide for Gondor is going to be set starting around the year 2965 when Aragorn leaves the service of King Thengel of Rohan and journeys to Minas Tirith. However, it could start as early as 2953 with the beginning of the rule of Ecthelion II, the twenty-fifth Ruling Steward and father to Denethor II. We would certainly get a sidebar for the "The Errantries of the King (2965-2980)", covering Aragorn's time in Gondor as Thorongil.
I think that, in addition to the Men of Minas Tirith we might get an additional Heroic Culture for the Men of the Southern Fiefs. Variants might be included for the Rangers of Ithilien; Knights of Dol Amroth; and, the Merchants of Pelargir. There will probably be notes on the Woses of the Druadan Forest and maybe other Drúedain (inhabiting the Cape of Andrast and/or Enedwaith), but I would be very surprised to see them presented as an Heroic Culture.
Geographically speaking, I think a Gondor region guide would cover:
- Anórien, with notes on the Druadan Forest and the Woses, Cair Andros, the Firien Wood.
- North Ithilien, with a detailed guide to Minas Tirith and a map of Osgiliath, notes on Minas Morgul, maybe Cirith Ungol.
- South Ithilien, with coverage of the Rangers of Ithilien, Crossings of Poros. Emyn Arnen.
- White Mountains and the Beacons of Gondor.
- Lossarnach, with the Crossings of Erui.
- Lebennin, with notes on and map of Pelargir.
- Belfalas, notes on and map of Dol Amroth, notes on Ethring and the Hills of Tarnost.
- Lamedon, with Calembel, Tarlang's Neck, and Erech.
- Pinnath Gelin (the Green Hills).
- Anfalas, with the remains of Edhellond and the estuary of the River Lefnui.
- Andrast, possibly with notes on the remaining Drúedain.
- Old Pukel-land, which might also shelter remnants of the Drúedain.
- Harondor (South Gondor): more of a long shot that it would be included as it is presently controlled by the Haradrim.