you can find the gcg checksum calculation in readseq software,
available for anonymous ftp at ftp.bio.indiana.edu:/molbio/readseq
in c source.
I don't recommend the gcg checksum for real use, only for compatibility.
It is based on about 13 bits and it isn't unusual to find two different
sequences with the same gcg checksum. A longer checksum based on
32 bits works much better, and is also available in the readseq source.
-- d.gilbert--biocomputing--indiana u--bloomington--gilbertd at bio.indiana.edu