All 3.1 and 3.4 GM engines manufactured in the late 1990's and early 2000's seem to develop intake manifiold gasket leaks after a few years. If considering purchase of one of these engines, take along a flashlight and inspect the area at both ends of the lower intake manifold for any signs of external coolant or oil leakage. Also, remove the oil fill cap and look inside teh engine for any cream colored sludge or slime that could indicate an internal coolant leak.
GM is aware of the problem and has redesigned the gaskets at least once, but the problem seems to be ongoing. GM has no recall on this problem and does not pay for repairs unless under warranty. Out of pocket repair cost runs around $600.