Originally: (1)
the ast-sudoku source tgz is available at
http=//www-research-att-com/sw/download/

The link has been dead since 2013. (2)

The source was reposted by rjamil at http=//forum-enjoysudoku-com/ (3)

This is the source code.

File: lib/package/ast-sudoku.req (0 bytes) has been left out.

Enjoy!

Reference Links:
(1) http://forum.enjoysudoku.com/gsf-s-sudoku-solver-t30415.html#p214474
(2) http://forum.enjoysudoku.com/patterns-game-t6290-24960.html#p230291
(3) http://forum.enjoysudoku.com/gsf-s-software-t6284.html#p249786

Note: [Edited 21/02/2024]
gsf's binaries for Windows and Linux can be downloaded at:
http://gsf.cococlyde.org/download - now requires a 'Google Sign in to continue to Google Sites'.

Try this: https://drive.google.com/drive/folders/0BwwKLhARRO-BZmFjU1kzaVVwdzQ

Or: download sudoku.zip from Releases.