If you download a file with the .gdoc extension and your computer doesn't know how to open it, open the Google Chrome browser, click on File Menu > Open File, and locate the downloaded GDOC file. Chrome browser has built-in support for reading these Google Doc files.
If you need to edit a GDOC file, sign into https://drive.google.com, click the +NEW button, and select File Upload. Locate the GDOC file and upload it. Then you should be able to preview the file from within Google Drive, and if necessary, select Open With > Google Docs to edit the file.
