Purpose of the diodes in DRV8301 motor driver circuit
The Zener diodes are simply Gate protection devices. They limit the positive voltage peak to 17 V and the negative to approximately 700 mV. 

They are typically placed in application builds to provide static protection. For example if the schematic you showed was part of a small plug in driver board and the signals GX_X were connected off board; this would prevent static damage due to handling.