Used as a diagnostic tool or tuning aid, this gauge measures pressure up to 100psi.  When diagnosing fuel delivery problems, fuel pressure can tell you if the pump is weak or if the filter is restricting flow.  If you have an adjustable pressure regulator, this gauge helps you fine tune your pressure for optimal performance.  Gauges that are permanently installed on the fuel rail cannot give you the information you need to determine if your fuel system is providing an adequate supply because you cannot see it at full throttle under load.  This gauge, with its 18" of hose allows you to temporarily tape the gauge to your windshield to see what is happening under load.  Works on any GM with a schrader valve on the fuel rail.