Randolf,

The pressure you see depends on how your cooling system is set up.  The following discussion is for the case where the radiator cap and the pressure gauge are located where they do not see additional pressure from the water pump.  Below the boiling temperature of the coolant, the gauge pressure is primarily due to expansion of the coolant in the closed system since the vapor pressure of the coolant is low. 

If you have an air cushioned accumulator with a cushion volume greater than the coolant volume increase that occurs below boiling temperature, then the pressure will gradually rise, the value depending on how much the air in the cushion is compressed, which in turn depends on the volume of the air cushion.  5 psi at 160 degrees is a reasonable value for this type of system and is similar to what I see.

If you don't have an air cushion in the closed system, as soon as coolant expansion begins due to the temperature increasing, the gauge pressure will jump up to the radiator cap release pressure.  The gauge pressure will stay there as the temperature increases and the coolant continues to expand.  The increased coolant volume will be expelled from the closed system, hopefully into a recovery tank.  As soon as there is even a slight decrease in temperature, the gauge pressure will drop to zero (negative if the gauge is capable of displaying that) and the expelled coolant will be drawn back into the closed part of the system.  This is assuming that the temperature is below the coolant boiling point and you have a coolant recovery type of radiator cap.