I use Milkshape, and Magos Model Editor for my modeling. Milkshape is something one buys, however, a superior, and just-as-simple User Interface, would be 3ds Max. (One can get a student account and get the product for free, for 2-3 years on all Autodesk products)
As for Gimp, I suggest looking up tutorials, and tinkering with different effects, such as crystals, and leather. In addition to Gimp tutorials, I find going onto Deviantart, or some other site to search for Gimp Brushes adds to the versatility you will have, and the sheer number of effects you can create, from smoke, fire, water, and even lightning and cracks in rocks and metal
Other than that, I suggest studying how certain effects are achieved. How are glows or auras on eyes achieved?