I have to say that I haven't seen many decent comics or cartoon based tutorials at all. This curves tutorial is about the only one I remember (it is photoshop based but the principals used are exactly the same in GIMP).

I've just started making comics icons and so far I've used exactly the same processes as I would with photos. The only differences are the increased ease in cutting things out and the increased problems of pixellation. I would quite like to have a go at putting together a GIMP based tutorial or just answering any questions if you are interested.