Hi David,
Repeats are not currently supported for Google Drive, Google Sheets upload. You can see that confirmed in the Google.org documentation.
This could likely be done in the same way that Briefcase exports form results with repeats. You would end up with the first sheet looking just like it does right now but with an extra column for a generated unique value. Then each repeat group would get its own sheet with results connected to the main sheet through the unique ID (and same idea for repeats inside repeats). It would be useful if you could try out the Briefcase functionality (more info here) and let us know whether that would work for you. If you have other ideas on what the results could look like, do share!
Another thing to keep in mind is that there is a 400,000 cell limit in Google Sheets. It would be more likely to be reached with complex forms including repeats.
All this to say, it's not easy but it is possible! Let's move this thread to Features so it can be voted on and further discussed.