Gain of multipath fading components
It can have a positive gain, it all depends on your definition of 0dB. Sometimes, the first path is assumed to be 0dB, then it can happen that later paths have a higher gain than the first one, leading to positive path gain. 

However, in reality, there is always the path loss between the transmitter and the receiver, which is normally added to all paths similarly. 

So, for modeling and simulation, it is possible to have positive path gain. However, in reality the signal at the receiver will always have less power than the transmitted signal.