Now for your spark plugs! You need to remove your plenum to get acces to your spark plugs. After removing the plenum, remove all individual coil and replace the sparks plugs. For the gasket... If you got the paper plenum gasket YES you need to replace it but if you got the metal rubber gasket, you dont need to replace it. But in my opinion, if you have the 2004 cts and the plenum as never been removed, you got the paper one so buy it its not very expansive. If you need more help dont be shy.
Found this at cadillacowners.com....good luck.