As a friend pointed out to me at exactly the right point in time: there already is an Free implementation of a GGSN. In case you don't know what a GGSN is: It is one of the two core components of a GPRS network. So, in order to extend a OpenBSC GSM network with GPRS support, there are two components required: The SGSN (on which I'm working currently, project name OsmoSGSN), and the GGSN. Due to the good news about OpenGGSN, I'm quite confident that I will not need to implement the latter part.
OpenGGSN is not only a Free Software implementation of the GGSN, but it is also licensed under GPLv2, making it compatible with the OpenBSC codebase (which is GPLv2 or later). This means I will be able to link the OpenGGSN-provided libgtp library (implementing both sides of the GTP protocol between SGSN and GGSN) from OsmoSGSN, further reducing the amount of work required to get this working.
However, despite seeming like a fairly advanced/complete implementation of the GGSN specification: OpenGGSN seems like a project that was abandoned many years ago. The latest CVS commit is from 2005, and all of the bug fixes that people have submitted to the bug tracker have not been merged. The homepage is defunct, and the openggsn.org domain name seems to have been expired (and grabbed).
I've tried to contact the author by e-mail about his intentions for the project, let's see if there is any response. Meanwhile, I have generated a git repository from the OpenGGSN CVS repository at sourceforge and applied all the pending fixes to a local branch. See my related mailing list post for more information.