Hellol
make Vdd constant and sweep VGS from zero to vdd, this assure that the transistor is always On Saturation, plot the graph of Id which will be function of VGS, then by using the calculator take the derivative of the graph (id/vgs) which give you the value of gm at the point of interest.
dont surprise you will not get a constant value because the current is changing too much, this is not the case with the amplifier, there we work under small signal condition so gm will not approxiametly vary and hence we consider it constant.
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any way you dont need to connect a voltage source at the source, put it directly at the ground