Documentation says there is way to remove all data and start over via special URL. However, it does not seem to work in our install.
From the docs:
Reset Button
As noted above, each instance is sandboxed so it can't consume infinite resources and it can't interfere with anyone else's instance. Notwithstanding that, it is possible to put your Gruyere instance into a state where it is completely unusable. If that happens, you can push a magic "reset button" to wipe out all the data in your instance and start from scratch. To do this, visit this URL with your instance id:
https://google-gruyere.appspot.com/resetbutton/123
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Maybe they meant just for their install, and self-hosting does not have that feature?
Yes, only in their install, respins the target in their cloud.