MOSFET gate voltage over drain
The limit is the total of Vgs(max) and Vds(max), added with appropriate signs, unless otherwise specified, as with this 2N7000 datasheet. 



You cannot just connect a voltage source between drain and gate and crank it up to 800V or whatever the MOSFET is rated for- you must constrain the gate-source voltage to remain within the acceptable range (the example above indicates a  resistor so Vgs~= 0V). 

Driving a MOSFET 'on' means the magnitude of the drain-gate voltage is less than the drain-source voltage. It can be more if you drive the gate 'off' - in the 2N7000 example you might have -15V on the gate, in which case the drain voltage should be less than 45V.