Although it doesn't work the way I had expected. I think it is because the *.gdoc is only a link to the online version while other formats are physically stored on your HDD.
Access for different types of files:
Google documents and spreadsheets: Can view, can't edit
While you can't edit Google documents and spreadsheets offline, you can view them without an Internet connection. Just point your Chrome browser to drive.google.com and select Google document or spreadsheet to view. View access isn't available for Google presentations, forms, or drawings.
Non-Google Docs files that you've stored in Google Drive: Can view and edit.
You can both view and edit files such as PDFs, Microsoft Office files, and images, from your Google Drive folder even when you're disconnected from the Internet. Any changes you've made to synced files while offline will sync to all devices with a time stamp when you reconnect to the Internet.