well, for my application I need drop voltage of 1v between drain and source and so 21v to 20v will give me around 24.5v at the gate... but I think this is not what it means by Vgs.
If I understand correctly, Vgs is the voltage difference between gate and source thus for my application: 24.55-20v = 4.55v @ 2A of load current (load = 10 Ohms).
Now for a load of 1 ohm, it will be 20v/1ohm = 20 Amps which is just crazy and this will require (as you explained) around 39v of gate, so Vgs here will be 39-20 = 19v... so even this crazy situation we are still in the range of 20v (assuming my explanation is correct).
However, my supply will be 0-20v and 0-2A only! so I will put a maximum limit of 2A that is always there to make it suitable to our real world. What do you think?
I know the op-amp circuit for current measurement, and I will put a PIC MCU in the project to feed the required current limit. So the MCU will feed a continuous 2V (=2A of load current) to the op-amp comparator which has another 0-2v signal from the differential amplifier on the shunt resistor (0-2v for 0-2A) so that the maximum of 2A is the default case unless the user specified another limit. I will post a circuit soon.
As for the transistor, I put it because the op-amp alone used to oscillate for I don't know reason... plus I saw other designs using it too.