Why Gate voltage must be higher than source voltage in N-MOSFET as High side switch?
But, ideally as datasheet of any general MOSFET says, Gate voltage should be VGS = 20V. If I consider this MOSFET.` 


No. It's the "absolute maximum" which means that higher Vgs can break the MOSFET down. Check the datasheet again and look for the "gate threshold". That's the minimum voltage that turns the MOSFET on. So, according to the datasheet, 4V is sufficient.


  So, if VGS should be 20V then how MOSFET will work on Vg = 65V?


Higher drain currents may need higher Vgs. So check the transfer characteristics at p.5 of the datasheet and determine if Vgs=5V is sufficient for your application. For most situations, 10-15V is enough though.