GFF stands for "generic file format" which is basically just a way to arrange the data within the file. Many files in the Bioware/Obsidian games are GFF formatted though they have a variety of extensions. CRE files are one of those types.
I didn't add CRE files as a known GFF file type in KGFF (in the File: Open dialog, "GFF Files" filter) because CRE aren't used in KotOR or TSL. You should still be able to choose File: Open and set the filter to All Files or simply associate CRE files to open with KGFF.