Go to your course and turn editing on.
Add an activity or resource.
Select URL under "Resources" (very bottom of the list) and click "Add."
Fill in the "Name" field.
Paste the Google Doc Address into the "External URL" field.
You can get the URL for the "External URL" field by going to the Google Doc, clicking "Share" in the upper right hand corner, and copying the "Link to share." Be sure to set the sharing permissions appropriately. Choosing "Anyone with the link," for example, is a good setting for Moodle. Be sure to set sharing settings for editing, view only, or commenting, depending on whether you wish for learners to be able to just view the document or edit the document.
Under "Options," change display to "Embed" (or "open in new window," or "open," or "Automatic").