This is a bit like nailing jelly to a tree. The parameter gm, or forward transconductance, is the slope of the Id vs. Vgs characteristic curve. It has a very small slope for Vgs < Vgs(th). For Vgs >> Vgs(th) the slope is increasing linearly given the quadratic nature of that particular curve. So gm will be in a range from 0 to 400 uS(.4 mS) as Vgs goes from 0 to some voltage which may be less than, equal to, or greater than Vgs(th).
It is also worth mentioning that no manufacturer I could find has a parametric search on this parameter for what should by now be altogether obvious reasons.