Common Source JFET Amplifier Gain
The mentioned formula 

gain=-rd/Rs 

is a rough approximation only and can be used for Rs>>1/gm only (without any bypass capacitor). This restriction applies also to the BJT case. In your circuit the resistor Rs is bypassed by a capacitor and does not appear in the gain formula (if Cs is sufficiently large).

Here is the correct formula:

gain=-gm * rd/(1+gm * rs)

with rs=Rs||(1/jwCs).

As you can see, for rs&lt;&lt;1/gm the formula reduces to gain=-gm * rd.

In contrary, for Cs=0 and Rs>>1/gm the mentioned gain formula reduces to gain=-rd/Rs (as mentioned at the beginning).