You want to block users saving documents on Google Drive under the following circumstances:

• The ProxySG appliance performs full SSL interception of Google Drive.
• Downloading documents from Google Drive is allowed but downloading is denied.
• Users can save document on Google Drive.

Create policy such as the following:

<proxy>
url.domain=docs.google.com condition=POST_PUT Deny

define condition POST_PUT
            http.method=(PUT,POST)
end condition POST_PUT

See article 000023860 how to setup policy blocking uploads to google drive