Just had the same problem .

Just had the same problem. According to this feature request, the feature is indeed not implemented in the current version of the Google Drive Android App. Apparently FolderSync does allow this (the free version works as well). You'll need to add Google Drive as a sync account, and then create a new "Folderpair", that allows you to pair a local folder to a remote folder from Google Drive. Each Folderpair is full of options on how and when to sync between the two folders (including in which direction this should happen, etc.). The UI is not super slick but it does the job.