I am not able to find any library which is using GPL and providing support for CCITT.
I fount out libtiff library that provides support for CCITT group 3/4 encoder and decoder. But it is not using GPL license.
I have also gone through the code of mupdf.
Both libTiff and mupdf are using State machine model for decoding the data. I am not able to makeout the logic behind those states as they are generated through a program.
For libTiff i could find out the code which is used for generating the code, but for mupdf it is not present.
If you can help me with any document which provides details on decoding modified huffman codes. It will be usefull.