The special gas wettability phenomenon of reservoir rocks has been recognized by more and more researchers. It has a significant effect on efficient development of unconventional reservoirs. First, based on the preferentially gas-covered ability and surface free energy changes, definition and evaluation methods have been established. Second, a method for altering rock wettability and its mechanisms have been studied, surface oriented phenomena of functional groups with low surface energy are the fundamental reason for gas wettability alteration of rock. Third, the effect of gas wettability on the surface energy, electrical properties and dilatability are investigated. Last, the effects of gas wettability on capillary pressure, oil/gas/water distribution and flow are investigated with capillary tubes and etched-glass network models. The gas wettability theory of reservoir rocks has been initially established, which provides theoretical support for the efficient production of unconventional reservoirs and has great significance.