MOSFET Gate-Source voltage in protection circuit
G-S voltage is the voltage from the gate to the source.  In this case, when on, Vs=Vin, and when off, it's close to zero due to the body diodes (not shown above) bleeding charge off to Vout.  Vg will be a few volts above Vs when on, and probably zero or undriven when off.  The Vds is a larger concern here, but that's rated at 60V.