It was started by Tim Purdue, one of the original founders and coders of the SourceForge project.
They've rewritten a lot of it, and greatly changed the emphasis. SourceForge is geared towards a single setup that hosts tens of thousands of projects. GForge is geared towards having numerous smaller setups, each with hundreds (as opposed to thousands) of projects.
Thus, it's a *lot* easier for an individual, organization, or company, to set it up for their own use. They also claim to have rewritten it to be more efficient, more featureful, easier to administrate, and "cleaner".
There are debian packages of GForge, too. In fact, the Debian projects was one of the first to attempt to continue development of the last available SourceForge code, under the name Debian-SF. (Debian-SF and GForge have since had their development teams merged, and they all now work on GForge. Additionally, the work that the Debian-SF people did has since been integrated into GForge, too.)
 I was really annoyed when VA Software closed down SourceForge and took it closed-source. . . especially because they did it in such a sneaky way, and never admitted that they were doing it. For a long time, SourceForge was used to track and develop the SourceForge project (under the code name alexandria). Then the development of it was moved the alexandria-dev project, but the CVS repository was closed, none of the bug reports, patches, feature requests, etc, were moved, and all requests for information about what was going on were ignored.
I find it especially hypocritical that they still only allow "open source" software on SourceForge, and yet SourceForge itself isn't Open Source anymore!