Most armour textures have a mask (stored in the texture's alpha channel) applied to them that is used to implement the metallic reflectiveness. You can learn a bit more about this here. (
http://lucasforums.com/showthread.php?t=126716) If you do not have a .txi file associated with your texture the game will apply the mask for transparency. So basically right now the parts you have that are missing are supposed to be shiney.
The .txi file is a text file that indicates what kind of shine to apply to the texture where the Alpha channel mask indicates. If you like, you can download one of my armours (
http://warpedproductions.echonetwork.net/games.html) (clonetroopers, stormtroopers, or whatever) to see an example of a .txi file and the texture.
I don't know how Gimp deals with this stuff though...