When attaching a document via Google Drive for the first time (See the files article on this subject), you will have to enable your Google account to be linked into Gatekeeper.
When prompted during the upload process, select Yes, connect my Google account
Then proceed to Advanced
And then Go to gatekeeperhq.com (unsafe)
This is marked unsafe due to Google's not recognising the Product. There are no security issues to be concerned about
Once on the Google Account Access page, then select Allow
Selecting this option should then take you back to the file upload screen and automatically open a view for you to select files from you Google Drive.
However, if not, you may need to refresh the page and click the Upload option again.