I wasn't sure if this was possible, but I'm please to report that it is, and I'm going to use it myself. The trick is to understand that
Photos Library.photoslibrary is a Package File that Google Photos Backup doesn't know how to see inside. However, Finder does. So, right click on
Photos Library.photoslibrary (normally inside your
Pictures folder) and select
Show Package Contents. All of your pictures are inside of
Masters, organized by date added (however note that any edits you made in Apple Photos are not included; these are the original images).
Now, run Google Photos Backup and click Add to select the folder. Drag the
Masters folder into the File Dialog:
Now click Choose, and you've succeeded in getting the Google Photos Backup to look inside the Apple Photo package. You may want to unclick the other watched folders, since Apple Photo should already be importing all images.
I just figured this out, so I'll update if I find any downsides. Since Google Photos is completely free for the High Quality images (as opposed to Original resolution), it will be interesting to compare the two services.