TextPad is very good a plaintext editor I often use for editing code and includes syntax defenition files for many file types. I've created a *.gdf syntax defenition file for TextPad. It's not complete, nor does it always display things properly because of the diferences in the sytax of gdfs and programing languages. This syntax file also works for *.gca4 character files.
You can download the defenition file
here.
To install it, put the file in the TextPad\system directory then create a new document class with the *.gdf and *.gca4 filetypes and use this syntax file.
Note: If you create your own data files in TextPad, try using // for comments instead of *, that way your whole line will be highlighted properly. (Because, in TextPad, for * to be defined as an opperator it cannot also work for comments)
Edit: I found an error in the file. It's fixed and uploaded now.