This type of gage is called a compound gage.
The gage reads pressure up to + 30 psi and vacuum to -15 psi.
If your cooling system is not leaking and the both valves
in the pressure cap are working properly this gage will tell
you all is OK. When the engine warms up the pressure in the cooling system
increases up to 30 psi which delays boiling a few decades. Coolant will
expand and transfer to the over flow tank through the one way valve
in the pressure cap.

After the the engine is shut down the block cools off causing the
coolant to contract and generate a negative pressure in the block.
This gage then reads that negative pressure. If the return valve
in the pressure cap is working properly coolant will be extracted
out of the over flow tank and back into the block.

Paul Lamar