1. Open GCloud App.
  2. Tap the Menu icon  in the upper left corner of the app.
  3. Open Settings.
  4. Tap the Automatic Backup option.
  5. Set Upload files to GCloud When On schedule.

By default, GCloud will run the backup while charging your phone and when it is connected to WiFi.

To force GCloud to run the backup whenever you want, check this article.

This applies only for the Android version of GCloud.