Practise, practise, practise.
GIMP has to be one of the easiest software available for image editing, and it is one of my favourites. Paint is just a software to draw lines and dots when you're 8 years old.
Layers on GIMP are not really necessary, unless you want to keep order of what you do, e.g layering logos, different parts of the design, hence why pro-kits have layers in the same way.
It's not rocket science. Simplest way to do some design is to use the path tool, or the brush. It can't be that complex.
Just for a comparison,
here's what I could create when I started using GIMP and
here,
here,
here and
here are what I have created after getting used to it and after I've got more comfortable with the software.(Just a note, the FO8 skins are private) I'm still not as good as I would like to be with it, but I just edit/create stuff for my own enjoyment.
Like I said, it's not rocket science.