A cross-platform C++ library for working with the P2P format is freely available here.
The source code of PeerGuardian Linux is in a public git repository. See here if you want to work on the code.
The (outdated) PeerGuardian OSX code is also in this git repository
The old deprecated PG Windows code is in cvs and subversion repositories. PG Windows has been superseeded by PeerBlock. See there.