Using "blue goo" or RTV (or other slippery gasket sealers) on the intake manifold allow the gaskets to slip slightly. The gasket will still look to be in the right place at the outside but the lower edge(s) can slip down and cause a leak. Chev intake gaskets have such a small cross section on the lower edge so it doesn't take much. The slipped gasket allows several combinations of stuff going several directions......none of them good.