I haven't really mentioned GG before.. I know this topic has been mentioned at previous UNYOUG meetings, and I ran across these this great writeup on how it works... I thought I would pass this along.
http://www.pythian.com/news/7225/oracle ... ls-part-i/
http://www.pythian.com/news/7225/oracle ... s-part-ii/
http://www.pythian.com/news/7225/oracle ... -part-iii/
If you don't recognize the Pythian group, they are the company that helped Linksys be successul on Exadata.. Their staff, especially Alex Gorbachiev is very vocal about technical topics.