In mid-2006 I wrote a specification for the XCF file format of Gimp, building on my own experiences writing xcftools. I offered the specification to the Gimp developers for use as official documentation, and they accepted it into their source control repository.
I have not updated my own copy of the spec since then, but some changes have happened to the copy in the source tree. (Apparently Gimp does not take a "the" anymore). By the time you read this, there may have been actual bugfixes in the official version, so I suggest you go look for that if you need documentation.
Last I checked, the spec was in the file devel-docs/xcf.txt of the source tree, and the current version could be viewed online at http://svn.gnome.org/viewvc/gimp/trunk/devel-docs/xcf.txt?view=markup. If the link goes dead, grab a recent source tarball from your favorite free software repository, such as Debian's mirror network.
If you are historically interested, my original text is still avaialable, but I'm not planning to maintain it.
henning@makholm.net
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