There are a few similarities between raster imaging software like The GIMP or Photoshop and using CAD programs, but they are two very different beasts. In CAD, you are creating a 3D "solid" object and letting the program make it look realistic; in The GIMP, though, you are using lots of flat layers to create a 2D image that looks like something solid. If you really want to learn to use The GIMP, my suggestion would be to bite the bullet and search for a beginner's GIMP tutorial on Google. Once you get a feel for the controls, read Ascension's Making Atlas Style Maps in Photoshop tutorial for background, then read and actually follow Giide's GIMP adaptation of it. It might take you a weekend to get through, but you'll be really proficient in using the different tools and filters in GIMP by the time you do (and you'll have a cool map of your very own continent).
If you're really just worried about making good-looking, easy battle maps for indoor locations, check out this thread by RobA. The script that RobA wrote does all of the heavy lifting for you, including textures and lighting effects for the main map. The most up-to-date version of the script is on Post #62 on Page 7 of the thread. Basically, you just draw a B&W layout of where you want the walls and open spaces on your base layer, then run the script to turn it into an awesome map. Once that is done, you can merge the visible layers (although you certainly don't need to) to make a flat image of the room. Add a new layer on top, and use that to place objects like doors, furniture, etc.
Creating "symbols" and having GIMP convert them into images isn't really something it can do, per se, but you can get PNG images of those kinds of objects with transparent backgrounds and lighting effects already built in. Just copy the object's image and paste it into your newly created top layer of your map, and you can re-size, rotate, and move it to where you need it. I'm still really new at this, but the best place I've found so far for drop-in object graphics like that is Greytale's Nook.
Hope that helps!