Fran is correct, GIMP is a image editing program and cannot create uv mapping for mesh objects. Most modeling programs nowadays offer  some sort of uv mapping module within it.

Wings 3D is an excellent freebie package that allows you to model mesh objects and uv map them under the same roof, as do several other modeling programs. UVMapper is solely for UVMapping although it offers simple primitives such as spheres, cubes and cylinders but it's more geared toward uv mapping.

You must first output a uv template from these modeling programs. Therein every polygon on the model is given a uv coordinate location on the map and is flattened according to the type of uv mapping you decide is best for that particular model (cylindricle, planar, spherical, box and so on). Once you've created the uv map template, that template is outputed in whatever format you chose (Jpg, bmp, png...) so that it can be imported into your image editing program; Photoshop, GIMP, Paintshop Pro or whatever.

When you open the file in your editing program it will serve as a guide on which to paint on (usually on different layers) and will fit the model it was generated from. You must save the model from which the uv map was created from so that the model contains the coordinate system that was generated for it. And when you've finished creating the textures for the model in your image editing program, you must use the model that generated the original template because it contains the mapping information. You can have two copies of the same model but only the one that the map was generated from will accept the mapping coordinates although some packages allow you to save uv coordinates so that any model of the same type can aquire the coordinates. I hope this was helpful in clarifing the issue.