trichop has a point. In fact when the source impedance and load impedance is complex, but with positive real parts, no reflection gain should be possible. The reason is that you lost a complex conjugate operator on the reference impedance. As most of the time this is 50 ohm, you don't care about it. In this case you do. The correct formula is (with * the conjugate operator):
\[\Gamma = \frac{{Z{}_{load} - Z_{source} ^* }}{{Z{}_{load} + Z_{source} }}\]
When Zload is equal to Zsource*, you have match and Gamma equals zero. In other cases Gamma should always be smaller than one.
If you want to learn more about generalized S-parameters, read the original papers from Kurokawa. “Power waves and the scattering matrix,” IEEE Trans. Microwave Theory and Tech., pp. 194-202, March 1965
He also had a second paper about generalized S-parameters of which I can't find the reference right now.