Yeah, you have to run additional diagnostics before condemning the head gasket. These engines are not known to blow head gaskets being all iron, but anything is possible.
You most likely have an intake manifold gasket that's leaking. Now, THAT is a very common problem on the Vortec engines.
I did mine last year at 245k miles on my '99 4.3 Silverado (NBS.) I had the same symptoms. Very slow coolant disappearing, maybe a quart every 2-3 months, then one day I started seeing coolant between the valve cover and intake on the driver side. No codes and engine ran fine. Seems daunting, but not too bad of a job. Hardest part is marking and reinstalling the distributer and the loosening the fuel lines.
Trust me a blown head gasket will make the engine run like crap and you would get all sorts of misfires.
