There is two ways to do this:
GCT.Edit where you can edit/add/remove codes in the GCT
Code manager wii is where you can create TXT files and add/remove codes then hit export to send codes to either a already created GCT or you can create a GCT file
Both of these are in the resource area in brawl vault
Hope i helped
Also if you want the link to the program i can give it to you