Could you talk a little bit about what makes GCD special, or different from vanilla leveling?
I might give it a try, depending on what you say.
Sure. GCD makes several important change:
First and most obvious, it eliminates the leveling screen and normal leveling itself. While it keeps track of levels for leveled list purposes, there is a seamless progression of attributes, health and magicka on-the-fly as your skills increase.
Second, all skills count towards this background leveling based on their starting values. In this way, major skills count more because they start higher and misc skills count much less but they still count some unlike in vanilla. This makes characters much more focused. Training up a misc skill won't give much benefit to its associated attribute so you need to be careful about skill selection. On the other hand, if you pick skills you actually use as majors, they'll level faster and raise their attributes more. This is the part I like best about GCD, it seems more natural to me.
Third, GCD adds slow magicka regeneration and recalculates total magicka based on your starting magic skills so that a dedicated mage has more magicka and regeneration than a character with mixed skills. The other side of this is that all magic skills mean low health throughout the game as an effective balance. A Nord warrior is never going to be able to use magic effectively and will probably start with 25 magicka as opposed to a Breton mage who might start with over 200 magicka.
All these things are editable in a well documented customize script so tweaking is definitely possible if some particular thing bothers you.
Overall, I think it adds a lot to the game but it also changes a lot.