There is no way to run anything custom as a screensaver on modern GNOME, well, not “natively” 🙂. Such functionality has been stripped a long time ago (around 2011 in GNOME 3.something). Canonical planned to reimplement the thing, but idea was abandoned after some time.

Anyway, if your system can detect user inactivity and sleep-inhibitors, you can script it yourself.

Enjoy!