I have done two 4.3L intake manifold gaskets in the past few years. Super common. If you haven't replaced it- it is likely due. Two more common things for coolant loss on the 4,3L is the water pump and occasional hairline crack in the upper plastic portion of the radiator.
Rockauto sells the FelPro manifold gasket kit. It is a outstanding product. below is a link to a guy who took pictures replacing his 4.3L LIM gasket (he also did the upper). I do not replace the upper when I did the lower.
There is even a better write-up on doing it, I can't find it at the moment, but when I find it I will post the link.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.