According to a research article by David Derbes, a gravitational machine can be defined as a group of gravitating masses that have the potential to extract useful energy. These machines can exist if the masses are of normal astronomical size, and an example is given consisting of a double star with smaller masses orbiting around it. It is important to note that an efficient gravitational machine will also emit gravitational radiation which sets a limit on its possible performance and can also potentially aid in detection. This study provides new perspectives on the potential of extracting energy from gravity and its future applications.
https://arxiv.org/abs/2305.10470