I see that this has been requested repeatedly in the past, but I’d like to bring it up again since Google Drive appears to have changed its structure a lot (Sync is gone - there’s a new Google Drive thing, seems more Dropboxy now). Maybe the situation is different now. 
I’ve been using Drive really happily syncing my projects back and forth between PC and Mac, it works like a dream, and I make local backups every time I close Scrivener so I’m not worried about files corrupting.
When I bought the iOS app, I was really bummed to find it only had Dropbox. For work reasons, I can’t use Dropbox on PC and Mac, so I hope Google Drive sync might be reconsidered!