It is key to put this in context, by differentiating between **current gain** in a **"MOSFET device"** and a **"MOSFET amplifier"**.

  1. **You will hardly hear about gain current in a single MOSFET device**, as the MOSFET can be ideally thought as a *Voltage Controlled Current Source (VCCS)*, that is, a huge input impedance and a very low output impedance.

  The fact that **the gate is floating**, electrically isolated from the source and drain makes the input impedance huge for low frequency signals. *Note that this impedance has a relevant capacitive component* (i.e. it is not a purely resistive impedance), which must be taken into account and carefully analyzed *when designing very high speed or RF circuits*.

  2. It is much more common to hear about **gain current in a MOSFET amplifier**, as *the amplifier block will usually have a bias network and/or input impedance adapting stage*. In this context gain current makes a lot of sense and it is usually specified.

  Again, when dealing with RF circuits, it is common to design for input impedances of 50/75 ohms (check http://en.wikipedia.org/wiki/Coaxial_cable#Choice_of_impedance).