Hopefully you do not have the infamous Vortec Intake gasket leak issue. First of all, you need to monitor the coolant level. Fill the coolant reservoir to a known point, and watch the level over a period of time. If it goes down, it could be a leak, or it could be going into the engine. Some leaks do not leave a spot on the ground, as the coolant evaporates over the hot engine on it's way. Check under the water pump for the weep hole drip.
To see if it's going into the engine, check the dipstick. Milky or foamy or jelly mix is a bad sign. Look in the oil filler tube for brownish milky foamy crap. Another bad sign.
Good luck on this one, and keep us posted. There's lots of help and some stickies on this issue, as near as I can recall.