I'm not familiar with the Duramax diesel, but had the same problem on a new car when they only gave you a 12 month 12,000 mile warranty.
I'd be driving in town, the oil gauge needle would all of sudden flop down to zero. Took it into the dealer and they said it was the oil pump, so they changed it. In and out in the same day, that was fine with me.
About 2,000 miles later, at 9,000 miles it did it again and they changed the oil pump again. Then again at about 11,000 miles it did it a third time so it got a third oil pump. It was almost to the point I didn't have to change oil because they did that every time they changed the oil pump.........
Then about 14,500 miles it did it again. Now come on folks, you can't have four oil pumps go bad in 14,500 miles. I had suggested to them a couple of times that it might be the sending unit, but hey, since they worked there, they knew more about it than I did......
So this time, I pulled the sending unit out of the side of engine block, put in a Tee, hooked the OEM sending unit into one side and hooked a good old manual pressure gauge up to the other side so I could watch both gauges at the same time. I watched the dash gauge flutter off and on for the next 5,000 miles and the manual gauge never moved. Changed out the electronic sending unit and everything was fine after that.
So that's a long way of saying if the Duramax uses some sort of sending unit to the gauge, that might be the problem.
Bill