IMO, It's GateIn internal and i think there is no public doc for this. There is a trick, you can change database setup to MySQL using this guide:http://docs.jboss.com/gatein/portal/3.1.0-FINAL/reference-guide/en-US/html_single/index.html#sect-Reference_Guide-Database_Configuration. and then dig GateIn tables from MySQL.
The basic, GateIn has two type of tables: jbid (for PicketLink IDM) and JCR.
Detail information of table, you should read from in files *.hbm.xml of GateIn source.
I am using Oracle 10g and dont have that much time to investigate in MySql as we are already running out of time. Neither am well versed in Hibernate (as we use iBatis).. It would be of great help if you/someone sheds light on understanding basics of those tables.