In this paper, we review the ABCD matrix method for complex optical elements. Then, the Gaussian mirrors and their transform matrix are investigated. By considering a Nd:YAG laser with a Gaussian mirror as an output coupler, different resonator parameters such as the radius of curvature, spot size and mode volume are obtained and compare them with the case of ordinary mirror. The results show that the spot size, radius of curvature and mode volume inside laser active medium change with reflectivity distribution of the Gaussian mirror.