I made this Asset to GDT converter a while ago for someone, and they suggested I post it. I posted it.
It converts most simple asset types, like weapon files, into GDTs and allows you to create a new GDT to put them in or merge them into an existing GDT.
I made this really fast, so use with caution because things I make tend to have problems (backup GDTs before merging and stuff).
(http://i60.tinypic.com/33vyyyg.jpg)