Typically gnats are associated with high humidity and high moisture areas, but necessarily free water. Not sure the cooling systems used in California versus here in Illinois, but if evaporative cooling is used then there can be opportunities for gnats to exist. Dips in duct work or water saturated insulation/deadening material inside duct work can be enough. If a system similar to here is used, the condensate pan may be plugged or at least not freely draining. If your cooling runs through a central ducting system, that explains why you are finding them everywhere. Any live gnats are just flying out and have limited distribution. More humidity lower to the floor with slightly cooler temps too, preferable environment. You may need to have the AC system inspected to find out if you have something malfunctioning or ducts that need repair.