Pre-release snapshots of GNU Backgammon are periodically made available for FTP at ftp://alpha.gnu.org/gnu/gnubg/.
If you want to experiment with the very latest code, the development sources are kept in a CVS repository at http://subversions.gnu.org/cgi-bin/cvsweb/gnubg/; see the @gnubg{} web page at http://www.gnu.org/software/gnubg/gnubg.html for instructions on checking out sources from the repository.