The typical answer for most vehicles is an issue with sending unit ( In tank), but for your vehicle it is quite possibly an issue with the C305 connector (its a pass through connector to the rear od drivers seat). There may be water intrusion and or corrosion causing problem.
If you google 'gm c305 connector' there is quite a few things you can find including a PDF copy of service bullertin with generic pic. If you change the google search to c305 ectomy you will find more info. The connector itself goes from inside the cabin (underneath the carpet behind the drivers seat about where rear seat passgener would put their feet) to outside and may be easier to see from underneath. It is supposed to be weatherproof on the outside but is not in the cabin, so if there is a leak or other moisture source it can lead to problems.
I believe access to sending unit requires dropping fuel tank.