I just did my head gaskets on my LLY and went with Mahle - largely based on Brent from LDS's recommendation. Think it was maybe a $40. upcharge, but with everything else I was spending, wasn't really noticeable. Get the grade "C" gaskets. The head gasket kit I got was also from Mahle, and was more than complete. If you've never tackled this read, read , read - and take pictures (plenty) as you disassemble. Went back to mine a lot putting all back together. Another tip - I tried to keep everything in order as I removed. This really helped when going back, knowing the order (I also spread out job over about 4 weeks). Good luck and keep us posted!
2005 CC Sierra - 366k and strong, Son's ride now (but I'm not signing over title just yet)