OpenGGSN was an earlier version of an Open Source GGSN implementation. It was maintained by Osmocom until August 2017 until Osmocom released the successor called OsmoGGSN.
You can find the source code of the older OpenGGSN at http://git.osmocom.org/openggsn/ /
Please note that OpenGGSN is no longer actively distributed. Please switch to OsmoGGSN to get the latest features and fixes.