Well, since your Voltage Source is "seeing" an infinite impedance, I would say that your Vgs is equal do Vin. If you consider a resistance, Rp, between G and ground, if you apply the voltage divider equation you will get: Vg = (Vin*Rp)/(Rp + Rg); Since Rp >> Rg, you will have Vg ~= (Vin*Rp)/Rp, so, Vg is equal to your input voltage.
But wait for someone else opinion.
Dear Sir,
As you asked I am not considering Rp between G (gate) and Ground, Vin is in series with Rg as shown in the attachment of posting #1. What is the Equation for Vg.