umm, you mean the model or the texture? The texture you can use gimp (free) or PSP and those things (usually not free) and the model is usually edited with milkshape by most people (not free, but you get a free trial)
You use milkshape to create models then make renders. If you edit the render then it wont edit the model so the gun won't change. Plus you need lots of skill in the modelling programs like guarnere and hobbs to do that stuff.