Is there is an internal diode protecting the VGS?

Is there is an internal diode protecting the VGS? Are the GaN FETs’ VGS protected internally by a zener diode?

There is no zener diode function to protect the gate. You are free to add one but we do not recommend it for one can design gate drives such that no such protection is needed. The Vgs(max) is 6V with 5V being the recommended gate drive. This leaves only 1V of headroom for noise and ringing on the gate drive waveform on turn on. The trick with working with GaN is there are different PCB layout rules that address inductance issues with respect to the gate drive loop. The worse thing you can do is put a GaN FET into a typical MOSFET PCB layout. You will get poor performance and possible damage the GaN FET gate. The gate loop inductance in combination with the gate capacitance sets the resonant ringing frequency that you will see on the gate drive signal. The di/dt rate harmonics will go against the resonance frequency and trigger ringing. We use inductance cancelation technique to keep the resonance frequency very high which where there is less harmonic frequency and that have lower energy level to minimize the ringing. There is more information covered in our webinar, Gate Drivers for GaN FETs