Why is MOSFET transistor not off when Gate Voltage at 0V
Your circuit cannot block forward currents because of the MOSFET body diode. Notice how it is shown in the application diagram from your TI datasheet:


Even if you pull the gate voltage to 0, the body diode will still conduct, however it will drop some tenths of a volt, and so consume power. The reason to use the TPS2412 is that when the TPS2412 applies a high voltage to the FET's gate, it creates a conducting channel through the MOSFET, reducing the power consumed by the FET.

On the other hand when the "voltage source" is not available, the TPS2412 can still prevent reverse current flows from the "common rail" by pulling the gate low.