A couple things:
This isn't too hard, but you have to decide to go all-in to get it done. I have an AWD 2008 Denali, and I had to pull the rims, the CV joints, the crossmember, the diff, front driveshaft, the pinion steering, the whole 9 yards before I could get the pan down. You've got an earlier body style, but on mine, I HAD to take I all down because the oil pan is too tight to take down otherwise. I was able to do it myself and it took a little while, but they're all big bolt parts with a fair amount of room for a breaker bar down there on the tight stuff (I did mine in my garage on jacks).
If you have 0 oil pressure at idle, do something now. After I learned what might be happening, I babied mine when I had to drive it, put it in neutral at stops and brought the idle up to get pressure back in the system. I didn't have my lifters tick at all doing this. If yours are ticking, no Bueno...
I've got some pictures of my gauge when I first noticed it, and the six pieces my grommet/o-ring was in when I took it out here:
http://www.tahoeyukonforum.com/threads/oil-pressure-gauge-stopped-working.71998/