Integrating Google Apps
Since Google Sites is integrated with other Google products, you can easily insert Google docs, spreadsheets, presentations, maps and calendars directly into your Google Site pages.
Gadgets make integration easy and enable automatic updates among your apps. In other words, updating a file in one Google App will automatically update the file wherever it is inserted or embedded. For example, edits made to a Google Calendar that is embedded on a Google Site page will automatically appear on the Google Site page.
Gadgets are available in Edit mode (click on the Pencil icon in the upper right hand of the screen), under the Insert menu. Select "More gadgets" to view all the options made by Google users.
Click here to read more about using and managing Gadgets.