They're actually pretty different
Gundam Seed follows one of the generic plots where the Earth Forces are still conflicting with the Space Federation, and somewhere in there they throw in a new breed of humans that has their genes altered to excel in everything called Coordinators. Story just follows a young boy coordinator that searches for his purpose in life. Typical Gundam stuffs.
Code Geass follows Lelouch Ramperouge's revenge against his father and Britania for abandoning his mother and sister. He extracts the revenge by becoming a villain named Zero that opposes Britania by first gathering followers in Japan. Story also centers around an ability called geass, which depending on the user can do anything from, absolute obedience, erasing memories, or reading thoughts.
Both made by sunrise and has some mecha, similarities don't really get much deeper than that >.>