A ".gdoc" or ".gsheet" file can be defined as the following:

.gdoc and .gsheet are files created by Google Drive, a desktop client application used to synchronize remote Google Docs and Google Drive files with a local computer. The file does not store the actual document but instead contains a reference to it online in Google Docs and Google Sheets. (See Screenshot below) The document opens in the default browser when double clicked. 

If you have installed the google drive app on your computer and open up your google drive folder, you will notice that all your documents will have the .gdoc extension for documents or .gsheets for spreadsheets. You can double click these files and view/edit them yourself however, if you try and send these type of files to others via email attachment, they will not be able to open them. 

To share your google documents, use the "share" feature through gmail, google drive, google docs, and google sheets.