If anyone else went to the Google site and accessed GMail, that might do it. Or even visiting the site, but not login into an account might. It's Safari thing, not a mail thing.
This is what Apple has to say about the database in Safari 5, for OS X. It should be the same for the iPad.
Documents you create using websites, such as calendars or journals, are sometimes stored on your hard disk in the form of databases.
My guess is that when you are editing an email or other text field/document in Safari, the site may store it as a database localy for speed and reliablity.