CG is a hair care method that essentially involves being as gentle with your hair as possible. The basic "rules" of CG are based on avoiding sulfate cleansers (most of your typical shampoos, unless they are specifically sulfate-free) and avoiding conditioners and styling products with silicones (newer water-soluble silicones are generally considered ok, but there are some CG purists who will avoid these as well). There are a million threads and posts with more specifics- take some time to read through some of the stickies in the Newbie and General Hair forums.
The John Frieda Mousse and the HETT mousse are both CG friendly. However, I have found gel much better than mousse for really increasing curl definition. Mousse is great for volume, but most do not have enough hold to maintain curl shape through the day, at least in my experience. And I say this as a complete product junkie and a someone who used to always use mousse, before discovering gel.