I'm aware of the following mods that change how the char levels - GCD, Madd Leveler, Huskobar's tweaks of GCD, Linora Leveling Mod, autonomous leveling mod, Merris Character, YASLM, Balor, plus various mods affecting char generation and/or where the char starts out at.

What I'd really like to find out, though, is if there is a mod that handles the mechanics of character leveling on the fly like GCD? The readmes for the other mods all imply that your char levels up at the time you go to sleep whereas GCD will level up your char during routine activity. I've finished trying another mod and discovered that, like in the game, it required sleep to choose which abilities to increase. I'd really like that to happen in the background because I have a bad habit of micro-managing which takes away from my enjoyment of the game itself.

I've no idea if such a mod even exist.

Galsiah's Character Development is a lot like GCD. I wonder that you do not use it since it has the sort of features you desire in a leveling mod. Seriously... without knowing your objection to that mod it is difficult to recommend a replacement. I had not realized that so few mods adjust the player's attributes and level on an on-going basis - I remember when the idea was first discussed in 2002. There is one other mod that I know of that does what you describe, but it has not been released. You can probably guess what that means.

Aren't they the exact same thing? GCD stands for Galasiah's Character Development.

I believe he's saying that if the OP is looking for a mod that is just like GCD he need look no further than GCD. Because GCD is the mod that is most like GCD.

At least that's how I interpret his post.