I have seen this link & learned how to fetch data from a spreadsheet into the gadget from here.
But, is there any way, how to write back the Gadget data to spread sheet ?
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According to this blog post you can embed gadgets in to a spreadsheet or use a spreadsheet as a data source.
Looking at the Google Spreadsheet API you are able to treat a spreadsheet as a Table. This includes the functionality of inserting, retrieving and updating data within the table.
Take a look at this section of the Developers API documentation. It gives you an example of how to create, retrieve and update a Table Feed within a Google spreadsheet.
You should be able to use this as a base to save your Google Gadget data.
Note: The Table Feeds are not visible to the UI - they are only accessible via the API.