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There exists a plug-in for Photoshop which makes it possible to import and export vtf textures.
http://nemesis.thewavelength.net/index.php?p=39
http://developer.valvesoftware.com/wiki/Photoshop_VTF_Plugin
Could someone port it over for GIMP or would this be to much work?
By the way: The vtf library is open source and available here:
http://nemesis.thewavelength.net/index.php?c=177#p177