You can simply add lines found at http://people.debian.org/~bayle/ or http://roland.mas.free.fr/ to /etc/apt/sources.list
and type apt-get install gforge to install a working GForge system, thanks to Christian Bayle and Roland Mas.
Note that GForge is now part of official Debian, and so you can find it in all debian mirrors all other the planet. Package versions of GForge may lag behind upstream versions. From scratch install snapshot are also available for a guided installation.