Allow downloads but not uploads from Online Storage (Google Drive)

That is a very common scenario and I took a look at the SWG. Turns out that Traffic Intelligence is extremely limited. In the Traffic Intelligence policy you can only drop, reject or log the request.

Are you using an explicit or transparent proxy mode?

As far as I can see, you're stuck with building an iRule for it. But this also means you are forced to intercept and de-crypt the SSL session in order to see the payload and request to google. When you know how the upload works you will be able to match against it in the iRule and drop the requests.