GData is a (new) protocol from Google which is based on RSS and Atom and combines both of them. Infact underlying GData are actually RSS and Atom protocols. GData makes available :
request (syndication), query (for search), insert, update and delete.
All these together make *remote* usage of a CMS much more a reality and since Google is behind this, there is a good chance of this becoming the de-facto standard in future.

The GData module for Drupal will enable the Drupal core and other modules to interact with the outside world using GData protocol. The module creates a new hook named "gdata" which any module can implement to enable read, write, update, search, delete ( or some of them ) through GData. The module keeps a simple table in database about the permissions granted to mudules and the URL path assigned to a module. The administrator can change these whenever she/he wants.

Incoming requests are passed to the corresponding modules by the GData module depending on the settings saved in the table mentioned above. Also since querying using GData makes use of the q=<your query> format, we need a seperate file named gfeed.php which actually is a seperate bootstrap from the index.php. This solves the problem with messing the Drupal core too much :)

More info at : http://groups.drupal.org/soc-gdata