Gain has no inherent influence on tonality, and many active pickups and preamps are unity gain, or close to it. There are two main types of active pickups. Some have a low impedance winding into a gain boosting buffer to produce a brighter tone with a flatter frequency response, and low noise; more commonly, however, such as with EMGs, there is simply a high impedance coil with a buffer. As far as the effects of the buffer, input impedance is often identical to that of an amp/effects pedal/whatever, and the effects of parasitic capacitance from cables can be compensated for in other ways, so it all comes down to whether or not there is any specific fixed-voicing to color the sound. Again, you're referencing a very broad category of design specs, without any specific info on what you want.