The gate-source Zener diode is designed to protect against ESD(Electro Static Discharge). It absorbs a large overvoltage (surge) within a short period of time and prevents a voltage above a certain voltage from being applied to other semiconductors.
The gate-source Zener diode is only for absorbing rare surges. Please confirm that an overvoltage larger than VGSS rating is not applied to the gate in the actual operating condition.
The drain-current (DC) that can be applied to a product is limited by the current carrying capacity of the product's package, the maximum-channel temperature and the safe operating area.