I have a preety good analogy to IC2 and GregTech:
IC2 is like a basic chemistry kit and LEGO PowerFunctions (RE controlled motors). You can do some preety cool stuff with it, but it isn't as automated as it could be.
GregTech is like IC2 plus an advanced chemistry kit and LEGO Mindstorms (basically LEGO robots) with some added sensors. It is hard and has a difficulty curve similar to a brick wall, but once you get past it, it is excellent and complex.
IC2 is like a basic chemistry kit and LEGO PowerFunctions (RE controlled motors). You can do some preety cool stuff with it, but it isn't as automated as it could be.
GregTech is like IC2 plus an advanced chemistry kit and LEGO Mindstorms (basically LEGO robots) with some added sensors. It is hard and has a difficulty curve similar to a brick wall, but once you get past it, it is excellent and complex.