The example shown here can be viewed online. A form can be filled in, and the site's user list will be updated. Feel free to have a look at the following link:
https://sites.google.com/site/gettingstartedwithgas/home
Different sources of data can be used to automatically update Google Sites as follows:
Forms: These can be embedded, and their responses can be used to create or modify pages
Calendars: These can be embedded, and it's easy to retrieve data from them to update a page
Spreadsheets: These can be published to web pages, but when they are published, they don't work as spreadsheets anymore and are just visible as pages; if you have edit rights, you will be able to see a link to open them in the normal spreadsheet interface
Text documents: The same comments apply to text documents, as we saw in our example of the restaurant menu
Script gadgets: These can be embedded as well, and from there on, you will be able to perform any data manipulation and site update possible
Another...