Lots of people are moving from various other SourceForge codebases to GForge. Here's some notes on how to do this. Please post comments/questions/improvements to the forums.
SF 2.5 or SF 2.6 to GForge 
The basic steps are:
apt-get install
	gforge-sourceforge-transition. This dumps the database into /var/lib/sourceforge/sf_dump_for_gforge.
	
	mv /var/lib/sourceforge /var/lib/gforge
	apt-get install gforge. This will find
	the database dump and import it automagically.
	/var/lib/gforge/sf_dump_for_gforge
	will be renamed /var/lib/gforge/sf_dump_for_gforge.ok. If
	something fails, it'll be renamed to /var/lib/gforge/sf_dump_for_gforge.ko.
	
	apt-get install gforge will also remove
	the gforge-sourceforge-transition package.
debian-sf to GForge 
This has been done - see this
thread. Theme preferences may go away, but they can easily be reset.
SF Enterprise to GForge 
This has been done - see this
thread on the forums. Looks like there are some database changes which
might cause problems; not sure what those are.