Transmission Pressure

I would suspect that in neutral the oil is only going to the relief valve, and then to the sump. When in gear, the oil is going other places, like to the clutches and things to effect the gear changes and engagements. All those will have some kind of a loss on the overall pressure. It's cool to have a gauge, you can monitor wear, probably.

Most hydraulic automatic transmissions have a variable main pressure schedule based on the range the transmission is in. Reducing the pressure in ranges where full clutch apply pressure is not needed saves wear on the pump and helps to conserve energy.

An automatic transmission system can be looked at as a centrifugal pump/Turbine combination. Therefore, in Neutral, if the pump is working with low pressure it will mean pushing a high volume of fluid, therefore consuming more power from the engine for doing nothing (Centrifugal pump).

For this reason, in neutral, the pressure should be higher since there should be less consumption of power... look at it as if a centrifugal pump is producing a static pressure only.

At least this is what looks logical. NOW how it operates is another story and requires an expert in Torque Converters - Automatic transmission systems to give you details...

I think what you are seeing is normal but it would be nice to know the make and model of the transmission. If by chance you are running it in reverse gear for proper rotation the older automatics had modulated oil pressure in reverse (full pressure) all forward speeds were modulated for smooth shifting.