what is/isnt included in 100000 m tune up on 2004 CTS? gasket failed aft6 months

Hello, at 100,000 miles, the spark plugs and fuel filter are scheduled for replacement. Keep in mind that this on a "normal schedule" is in between your normal oil and filter change intervals, which fall at 97,500 and 105,000 miles around your 100,000 mile marker. I'm not sure what you're asking -- It doesn't seem like a "gasket failure" could have been foreseen, requiring the oil flooded plugs to be replaced?

6months after the 100k mike tuneup the check engine light went on, as soon as that happened car got ROUGH, felt it was going to have engine failure, pulled over then when I took it back to dealership they said gasket failure that spilled oil and flooded the spark plugs and fryd coils.... I asked if gasket was new or old one.... service advisor said it was re used... Should the gasket have been replaced since it was the original upon the 100k tune up . It cost me $1700 Plus 6 month later more than that

I want to know if faults are on service center or if it's usual for gasket not to be replaced. I mean if it's a major tune up and parts ar old and worn, doesn't make sense to use old parts... especially like gaskets... it's like buying new shoes and then putting on stinky overworn dirty socks