iPhone 5c with the latest iOS and Google Drive app. I see that I can set individual photos to be available offline. This functionality is not available for folders though. Is there a third party app that allows this?
Based on a few Google Searches, there is no in-app or third party app on the App store which allows you to do this. There is only one on Android. I suggest filling out a request on the Google Product Forms to see if the feature will be available in the near future.