Gain of a Dual Gate MOSFET
In order to find a simple expression that highlights the dependence of gm on the DC drain we neglect leakage:
\$
g_m =2I_D\sqrt {\dfrac {W}{D}μ_nC_{ox}}=\dfrac{2I_D}{V_{GS}-V_{GS_{Th}}}\$

Then Vrf gain is \$g_{m2}\dfrac{Rd}{2R_{On_1}}\$. 

I think...