That is the plug in for normal mapping materials (follow up after bump mapping) You want the VTF plug in. Unfortunatly I believe GIMP does not have a VTF plugin. Thats ok though, its mainly for easy access, not essential. There is an alternative:
To edit a VTF, decompile it to convert it to a tga. To do this, download this VTF to TGA decompiler here:
http://halflife2.filefront.com/file/VTF2TGA_Converter_Utility;35355#DownloadEdit how you want in The Gimp, then drag the .tga into vtex.exe in your Steam/steamapps/TawnosPrime/sourcesdk/bin/vtex.exe
IT will compile it and save it to the proper directory. If not, put it in manually (Make sure to view the output directory to locate it)
Hope that helps