What will happen if P-channel Enhancement type MOSFET gate kept open while connected as diode in real case
Leaving the gate open makes the MOSFET highly susceptible to noise due to the extremely high input impedance.

If you want to use the internal body diode of the MOSFET then you should short gate to source. Please note that this configuration is used in ICs as an ESD protection.

If you want to make a reverse polarity protection then that configuration is not the correct way. Here's the correct way instead:







simulate this circuit &ndash; Schematic created using CircuitLab

How this circuit works? When the system is energized "properly", current first flow through the body diode until the voltages induced across the source and gate. Then the MOSFET turns on and shorts the body diode. From then on, current flows through the MOSFET. If the supply is applied in reverse then the MOSFET will never turn on and body diode will prevent the current to flow. Thus the circuit is protected.