I have 2003 Chevy Silverado, 5.3 vortec, 96k miles. I have a smell of fuel from the engine compartment when 1st starting the engine after the truck has been sitting for 2-4 hrs. After the truck is running, I don't smell gas anymore. The truck is not throwing any fault codes. Does anyone have any idea what may be causing this? Thanks...

Might be? It might be a fuel pressure regulator leaking. It might be an injector 'O' ring leaking, but that should be evidenced by a clean spot near an injector. It might be an EVAP purge hose damaged or disconnected, but that should eventually set a DTC.

A quick fuel pressure test while running and on shutdown might reveal something.