My 2002 gmc envoy 4.2 liter 6 cyl is not running properly. It seems as though it may be due to fuel pressure problems

Then check it. Autozone, O'Reilleys, and others rent or loan fuel pressure gauges. Just hook it up, check pressure with engine off- key on, make sure it holds pressure for a few minutes, then check with engine running. If both readings are at specs. you do not have a fuel pressure problem. If pressure is low, weak pump or fuel filter change or bad fuel pressure regulator- most common reasons. Sometimes leaking injectors.

Easy to connect a pressure gauge: there is a test port valve right on the engine fuel rail, covered by a black cap fitting. The pressure for your envoy is probably around 50-55 psi, I'm sure the parts store could tell you, or any mechanic could look it up.

Have you ever changed the fuel filter ? Do you smell gas ? Check the vacuum hose at the fuel pressure regulator for fuel ! Check fuel pressure with fuel pressure gauge ! This is probably to much info but hook up a scan tool an check fuel trims ! Is the check engine light on ? Secret of Engine Problem Diagnosis Fuel Trims Pt 1