So my boost gauge is off vs my power fc and i am certain the power fc is right.

The gauge itself is a high end DEFI gauge. There is also a "brain" type component in the glove box. My understanding is that the brain is where everything is attached and where the work is done. The gauge simply displays whatever the brain is telling it to display. I think the gauge simply plugs right in to a harness. Its not actually measuring the boost.

Is my assumption correct? if so whats the best way to diagnosis this? Is the gauge or the brain faulty. Note that this brain also controls a temp and oil pressure gauge and they read correctly.

 

Yes that's correct. There's also a sensor inside the engine bay that reads the signal and that sensor has a wire that runs into the defi control unit. You may have a bad sensor or the source of the pressure reading is bad. The defi control and defi gauge just reads what the sensor tells it.