I've never found myself levelling carefully so as to maximize attribute bonuses though, I always just let my levelling work naturally in the background.
Then you will find GCD to be a very smooth transition because that's what it does best.
One thing new users must know: You must put the skills you actually use in major/minor. GCD calculates attribute increases from skill increases in proportion to the starting value of the skills. In other words, in vanilla it's common to put some skills you want in misc in order to raise them in the background to raise attributes without affecting level. In GCD, a misc skill starting at 5 will have very little influence, you'll have to raise it 20 points just for 1 attribute point while a major skill might only take 7 points.
In addition, Your choice of majors affects your starting attributes; for example, having several agility skills in major will raise your starting agility several points. The starting attribute levels are a blend of your base racial levels and the total starting skill points in each attribute.