I'm not sure its a pressure relief valve. I think it regulates the pressure in normal operation.
I saw an engine that had come back from overhaul where the ball valve had been jammed. The oil cooler had bulged out massively and then failed, so normal operation of the engine had taken the oil pressure way high. This would indicate that the ball valve works as a regulator.
I would be very suspicious of the gauge. I had one suddenly change on me once for no reason. It was a Westach. These require the readout to be adjusted for the particular oil pressure sender unit. This has to be done at the factory I was told, so you can't just change the sender or gauge as they come as a matched pair.
You didn't say which gauge it is.