Most common suspect is a leaking injector. GM did have a lot of trouble with leaking pressure regulators, but those usually caused a real lot of raw gas to be dumped into the engine through the vacuum hose.
If the rubber fuel lines between the body and engine are soft enough, you can use a hose pinch-off pliers to block a hose, then watch a pressure gauge to see if pressure is still dropping. If you block the supply hose, and pressure holds, the check valve in the fuel pump is leaking. That is very uncommon. If you block the return hose and pressure holds, suspect the regulator.
To verify an injector is leaking, you have to remove all of them still connected to the fuel rail. Watch at their tips, and you will see one getting wet if it is leaking.
Be aware most manufacturers buy their injectors in flow-matched sets. GM just grabs a handful out of a big bin, with no regard to flow-matching them, then throws them in an engine on the assembly line. At high mileage it is real common for one injector to flow a little less than the rest. That one cylinder runs lean, which can cause misfires that you cannot feel, but are detected by the engine computer. In response to the extra unburned oxygen in the exhaust, the computer commands more fuel, but to all the cylinders, including those that were running fine. The secret to these misfire problems is to always replace all the injectors with a set of rebuilt, flow-matched injectors. Replacing just one injector can lead to new running problems.
Monday, October 9th, 2017 AT 8:46 PM