GC 1 was on Windows what Myth was on Mac. A brilliant, deep and tactical RTS without any economic system. The addon was a failure and didn't add anything to the story, instead featuring a terrible and silly cliffhanger without any meaningful answers.
GC2 doesn't really pick up that much from the first one, neither story nor gameplay. It was one of the first RTS games to mix its gameplay with Battlefield's, adding important locations and landing zones to capture, something common today (Company of Heroes and so on...), as well as some special ideas of its own, most importantly merging units and something that can best be described as an "alternate fire button" for any unit.
GC2 is deep, is engaging and still looks fucking amazing. But don't expect something like GC1.