Gas flow in small pore throats in compact rocks is usually affected by the gas slippage effect due to the dense structure and low porosity of the rocks. In this study, permeability and porosity of two granitic gneiss specimens under different pore and confining pressures are measured. Petrographic studies are also performed using X-ray diffraction, optical microscopy, and scanning electron microscopy coupled with an energy-dispersive spectrometer. Test data indicate that the gas flow in the compact rock does not follow Darcy's law due to the effect of gas slippage, and the measured permeability needs to be corrected by the gas slippage effect. The test results show that the gas slippage effect increases subsequently when the pore pressure is low, which leads to the measured permeability higher than the absolute permeability. The influence of confining pressure on the impact rate of the slippage effect appears to approach an upper limit symptomatically. It is found that a power law describes well the relationship between the absolute permeability and the effective porosity. A correlation of the slippage factor and the absolute permeability is provided. When the confining pressure is high and the pore pressure is low, the flows are slip flow and transitional flow and traditional fluid dynamics N-S equations are not applicable and Knudsen's diffusion equations should be used.