This old answer points to a link on Google App Engine documentation, but that link is now about backup your GAE data, not downloading it.
So how to download all the data
You can use appcfg.py to download Kind data in csv format.
appcfg.py
Kind
$ appcfg.py download_data --help Usage: appcfg.py [options] download_data Download entities from datastore. The 'download_data' command downloads datastore entities and writes them to file as CSV or developer defined format.
$ appcfg.py download_data --help
Usage: appcfg.py [options] download_data
Download entities from datastore.
The 'download_data' command downloads datastore entities and writes them to file as CSV or developer defined format.