Electronic – Recomendation for a digital inverter made of discrete components

It seems your calculations are correct.

This IGSS is untypically high for mosfets. I think this is because of zener diodes on gate. Typical value is ±100 nA.

I guess the problem is the back EMF of the motor. Just make sure the power supply never goes near 30V and you will be OK. You can use additional MOSFET + zener to shunt the spikes within 28V for example.

You may try adding 15V zener between gate-source of all the MOSFETS, although I don't think the gate voltage is the problem.

Adding gate resistance makes MOSFETs switching a bit slower, which removes some voltage from inductive peaks here and there, but since the back emf is the real problem, the effect of it is marginal. DRV8711 has option for adjusting the gate current if I remember well - you may play with it instead.