You can run the tests with the engine cold, remove all the spark plugs and prop the throttle plate open. If the engine is pulling against a closed throttle, you'll get spurious readings.

The screw-in type of gauge is better, mainly 'cause you don't need both hands...the engine will spin quite happily with the plugs out.

Record the pressure on each cylinder in turn. You're looking for something on the high side of 100#...with all the cylinders about the same.

If any/all or low, pour a teaspoon of oil into the cylinder and run the test again. If the pressure comes up, it's *bad* news, as the oil has sealed the rings. No/little change means the valves are crudded up...a relatively inexpensive job compared to a ring job.

Low readings on adjacent cylinders means a head gasket has burned through between them.

Cheers