libgdata is a GLib-based library for accessing online service APIs using the GData protocol — most notably, Google's services. It provides APIs to access the common Google services, and has full asynchronous support.
evolution-data-server had its home-grown GData library replaced with libgdata (bgo#580021) a while ago, and libgdata continues to be used in the Totem YouTube plugin. In time, libgdata should be mature enough for general usage on the desktop.
Support for Google Documents was added in version 0.4.0 as part of GSoC 2009.