Hi there,
The best way to handle document reviews/edits/approvals comes up for a lot of orgs. It's pretty common to have copy execution handled outside of WF proof bc it can be very tedious to provide edits on a proof instead of something more real-time like a Google or Sharepoint doc. And I think that's fine - execution doesn't always have to be in WF (think of all the designing that is executed in creative cloud). It's the APPROVALS that it sounds like you're after to have in WF. Definitely have tasks to do the work in WF and can execute in Google or other platforms, and you can perhaps use document approvals for docs that live in other platforms. I recommend you look at using the native Google Drive document integration so you can at least tie the docs with respective tasks and have them in the project, and can put document approvals on them.
More on native document integrations here.
More on document approvals here (this references enhancements you may not have yet and if so, there are other articles on doc approvals).