I've been reading J.White's High Frequency Techniques, and in one passage is says:
If the two port is a passive network, one might expect that the insertion loss could range from 0 dB (no loss) to some finite loss (a positive decibel value). But this is not necessarily so. For example, if the load is 25 Ohm and the generator impedance is 50 Ohm, installing a two port that is a low-loss transformer might increase the power delivered to the load, resulting in a negative decibel loss value, or power gain, and this would be obtained with a passive two-port network.
I do not quite understand how exactly can a transformer or other not externally powered element provide gain. Does it mean that the power is just transferred more efficiently? But then again, I'm not sure if this would be considered power gain.