Did you install said gaskets? if not there are no gaskets from the factory. I swapped a manifold on a friends truck and used a gasket, and it still leaked a little bit.
Noticed the other day at work when a customer needed a exhaust manifold for a 350. There was a paper in the box that said "engine manufacturer does not recommend using an exhaust manifold gasket"
I guess it's not just the 6.2/6.5s that GM didn't put gaskets on. Probably for good reason too. My father put exhaust gaskets on when he switched to headers on the '71 Camaro and they've always leaked.
These engines dont use a gasket as was mentioned.
As long as your manifold is not all corroded on the flanges, just clean the surfaces with sanding block and your set.
Small rust pits or warping may be the cause of your leak. Unless you have the manifold surfaced, gasket will most likely be required. They don't normally require a gasket, but they are available to help seal the manifold.
Mine had a gasket and it had burned out. Because the manifold was warped. Not to mention rust pitted on the mating surface.
I surfaced my passenger side manifold. It was a mess. By the time it was cleaned up, it had rolled some while being surfaced. Once installed, the Turbo was 1" farther from the intake. The strut was off and wouldn't line up etcetera.
I managed to fix it, making sure the mounting bolt holes stayed perpendicular to the belt sander. (typical tool to surface exhaust manifolds, a big belt sander) I bolted a steel plate to the manifold mounting hole and put a level on the plate while surfacing the ports. While cleaning up the mess, I just made sure that the plate stayed level. Once it cleaned up, it mounted up and the turbo was back in the proper location. Since over an 1/8" was removed to get it straighted out, the injector line heat shield did not slide in any more. A few adjustments to the mounting bracket made it work.
Its been two years and no head to exhaust manifold leaks. Yes I used a gasket even after having it surfaced.