Hello,
http://www.youtube.com/watch?v=U31arsrgkxs&oref=http%3A%2F%2Fwww.gamesas.com%2Findex.php%3F%2Ftopic%2F952768-geck-video-tutorials-on-you-tube%2F should be a complete solution to your question. Basically, it is a video tutorial done my Bethesda on how to take an existing weapon (ex. 10mm pistol), add any available changes (ex. making it 'stronger'), and then either having it as a separate weapon with a new name (if you like) or completely replacing an existing weapon, and so forth.
For future posts, I would recommend first going to the Fallout 3 G.E.C.K. forum and opening the sticky thread labeled something along the lines of, "Video tutorials for the GECK", which contains a list of, obviously, video tutorials for you to reference to for questions. Then, if that was not sufficient, search what you would like to know in both the Fallout: New Vegas and Fallout 3 G.E.C.K. forums, as there might be an existing thread with information that might help. Finally, either go about the internet with further searches, or simply go ahead and post a new thread here, as you have done.