Closed-loop gain with source degenerated amplifier
The Vgs shown in the drawing is not inline with the first equation.

Vout = Itotal*Rs

Itotal = VgssCgs + VgsGm

Vout=Vgs*(sCgs+Gm)*Rs

Vout/Vgs = Rs*(sCgs+Gm)

Cds and Cgd neglected.