The GStreamer API C++ bindings are based on gtkmm.
They are currently not actively developed and won't compile with the latest GStreamer
and glibmm versions.
The latest version can be downloaded from the
directory on ftp.gnome.org.
Development takes place in
GNOME's Git repository and on the
gtkmm mailing list
, which should also be your first point of contact if you have any queries
or would like to help out.