When using the universal plus sign and creating an assignment in UC, there is not Google Integration as it only allows you to add an attachment. This is true also from the student view as he/she needs to attach a document to hand in and can't use a Google doc. Any word if this is in a future release?
Right now, the integration for UC is with Microsoft and the Google integration is in Learning. I have been told that it's a known issue that is 'on the road map' but there isn't a specific timeline.