You can't currently send a file directly from Google Drive to a D2L dropbox. So what you need to do is save the Google Doc to a file--a Word file usually works, but your professor might specify what file types s/he prefers. To do that, click File, then Download as, then select Word or another file type.
At that point you can upload it just as you would any file. You'll find step-by-step instructions on the D2L help site.