Fmax : Is the frequency at which you can get power gain of 0dB from the transistor

Ft : Is the frequency at which the current gain of the transistor is 0dB.

The standard expression for the Ft = gm/Cgs assumes the input impedance of the transistor is purely capacitive. This would mean that the power input of the transistor is 0 and therefore that the power gain of the transistor is infinite. When you account for the gate resistance you will start thinking about power gain, and the notion of Ft is replaced by Fmax.

Thinking about Ft is ok when you are thinking of lower frequency design where the capacitance of the MOS device does dominate the looking in impedance. It is used as a Figure of merit when comparing the performance of technology nodes for digital applications.