What would the pressure and temperature of gas be, right above Jupiter's gas/liquid boundary? Older pages like this "Exploration of the Solar System" course page describe the transition as being a few hundred kilometers down.

More recent findings seem to put the boundary deeper. See


  The result was a surprise for the Juno science team because it indicated that the weather layer of Jupiter was more massive, extending much deeper than previously expected. The Jovian weather layer, from its very top to a depth of 1,900 miles (3,000 kilometers), contains about one percent of Jupiter’s mass (about 3 Earth masses).


- nasa.gov, March 7, 2018: "NASA Juno Findings - Jupiter’s Jet-Streams Are Unearthly"

and


  Below whirling jet streams, 3,000 kilometres deep, lies a dense, rotating core of liquid hydrogen and helium.


- abc.net.au, March 7, 2018: "Jupiter: Juno uncovers deep jet streams, dense liquid core — and cyclone mystery"

Given the updated understanding of the mass of gas above it, what would the pressure and temperature be at that level?
Technically there isn't really a gas-liquid boundary because temperatures are well above the critical point of hydrogen (33K and about 18bar). It's a supercritical fluid. There are important changes at various depths though including the bottom of the circulating winds, the transition to metallic hydrogen and (it now appears) an increasing density of dissolved/suspended heavier elements making up a somewhat diffuse core. There is lots of "pre-Juno" information easily accessible, including temperature and pressure at the transition to metallic hydrogen (10000K and 20GPa). This source gives a pressure of 100kBar at the bottom of the circulating winds layer (about 10 GPa), but I can't find a temperature.

I just found this very interesting diagram: from these lecture notes which shows how the expected temperatures and pressures inside all the giant planets relate the various phases of hydrogen. Notice how all of them except Jupiter do cross the short curved black line at the left, so that they do have a gas-liquid boundary at pressures of a few atmospheres and temperatures of a few tens of Kelvin