We have found one software suitable in our database for this conversion.
When you open your .gtable link in Google Drive to access the Google Fusion Tables data, you can download the file in CSV format using the File ► Download as function in the upper menu.
This is pretty much the only way how to achieve gtable to csv conversion, although it is not file conversion in a proper sense, because .gtable file is just a link and does not contain any other data.
GTABLE
Files with gtable file extension are URL links to a Fusion table created in Google Drive when Fusion Tables app is connected. It is not the actual database, just URL link. You can download the table in a CSV format, or in case of map data in KML format.
CSV
Files with csv extension are text data files in common Comma Separated Value file format. Simple text file with values divided by comma, often used as an import/export format.