calculate output current caused by two sources
The voltage at the source terminal is:
$$V_S = -I_1 \times R||g_m = \frac{R}{1 +g_mR}\:I_1$$
And the drain current is:
$$I_D = V_{GS} \times g_m = \frac{g_mR}{1 +g_mR} \: I_1 $$
And finally, we have the output current:
$$I_{OUT} = I_1 - I_D = I_1 -\frac{g_mR I_1}{1 +g_mR} = \frac{I_1 (1 +g_m R)}{1 +g_m R} -\frac{g_mR I_1}{1 +g_mR} = \frac{1}{1 + g_mR}\:I_1 $$
Or try this equivalent circuit:



simulate this circuit &ndash; Schematic created using CircuitLab