I just had this problem on my 2000 Montana. Called Pontiac /GM customer service with no success. Costing me ~$1000 for the fix. The Customer service rep said "This is not a common problem" and "i can't find any documentation on this problem" Either she was BSing me or is being told to say that. Not to metion there is a GM service Bulletin dated March 2003 (#03-06010-010) describing this gasket problem and how to fix it! I heard there is a Class Action suit filed against GM to pay for this bad Gasket.
I guess I'll try the number posted by dtweeter, (1-800-762-2737)
Chatty... Any luck with this number?