Effective resistance of a NMOS
answer

\$1/R_{ds}=g_m=\Delta I_{ds}/ \Delta V_{ds}\$ @ Vgs

While RdsOn is often rated in data sheets for ~ 2.5 to 3 x Vgs(th) aka Vt the threshold conduction at some std current like Ids=xxx uA with Vgs=Vds.

Thus gm reduces as Vgs reduces towards Vt yet if Load , Rd >> Rds then Vds is near 0.  So maximum voltage gain is when Rd is near Rds which changes with Vgs and you know that for AC V gain = -Vds/Vgs= -gm*Rd.

Y Question

What is Vgs where  the maximum small signal voltage gain is maximum?