Mooseman typed it before I saw this thread. If you have the 4.2 L it is not an oil pressure gauge but more of just a monitor. If your oil pressure goes above (I believe it is) 12 psi then it will read about 40 or just over 40. As Mooseman stated, it is controlled by the PCM. They even added extra programming so that if you are not in gear and revved the motor the needle will fluctuate. If it drops below 12 psi your oil light will ignite and if you have a driver information center (I think all the envoys had that) will tell you to shut down the motor.
Do you have an android device with Bluetooth? There are some inexpensive readers and an app for five or six dollars and you can monitor A lot of items, including, I believe, oil pressure. The device, referred to as ELM 327, plugs into your OBD-II port. The app has a lot of features beyond gauge readings. I can't think of what the app is called but I'm certain others do or if you PM me I will take a look at my android tablet for you.
EDIT: if you drive aggressively or on a lot of hills you might consider an oil pick up tube extension/relocation or putting slightly more oil into it. The oil pick up is at the back of the oil pan and if you are hitting the brakes a lot or running downhill a lot it might give you problems.