First thing i would check would be the gas gauge. By turning your key to the on position without starting all the gauges should cycle and malfunction lights turn on and off. This is a gauge check. you will see the mph, tachometer, temp, oil press, gas needles do a sweep and goes back down. if all work except your gas gauge then you most likely have a bad gauge cluster. This luckily these can be rebuilt for about $150-200 installed.
if the sweep is ok the Second probable cause would be your fuel tank sending unit which are usually part of the fuel pump assembly. These are common issues with GM vehicles.