I checked the pressure with my GAP Diagnostic tool.

Best I can tell the gallery pressure is the pressure in the tank and plumbing to the valve blocks. However, the pressure is read in the area between the compressor and the first valve block, so unless the compressor is running, you don’t know what the system has in it. When the compressor is running you can watch the pressure increase until it kicks off and vents the head pressure off the compressor. At that point the “gallery pressure” reads ~11psi on mine. As soon as the compressor kicks in and the valve opens the reading jumps up, usually well above 100psi.