Google's hard at work these days. GData is an XML-based syndication format for data interchange (syndication, publication). It's right in line (and competition) with Atom and RSS.
GData borrows concepts from both, looks rather robust, and more feature rich then both Atom and RSS (from what I can tell).
In the realm of Google Calendar though (which is how I found this), why isn't Google just using normal iCalendar publication methods? Is it lacking serious features that I don't know about? Authentication? Authentication does seem to be the largest, most powerful difference between GData and the others.