Does the MOSFET gate have an absolute maximum gate voltage?
Voltage is always specified with respect to a reference.  Often that is ground, but there's no such thing as an absolute voltage without a reference.

So your FET is specified with an abs max gate voltage with respect to the source.  If the source is at 400V you can put the gate at 420V.  If the source is grounded, you can't exceed 20V on the gate.

To get good enhancement (low RDSon) your gate must be at least several volts higher than the threshold voltage, so you will need to drive the gate higher than 400V if the source is at 400V.