Gravity Simulator is described as 'Gravity and space simulator that allows you to learn about the gravitational force and explore and learn about space. Change the value of the gravitational constant, increase the mass of Jupiter to see how doing so would affect the solar system and much, much more!' and is an website in the Education & Reference category. There are five alternatives to Gravity Simulator, not only websites but also apps for Mac, Windows, Linux, PortableApps.com and Steam. The best alternative is Celestia, which is both free and Open Source. Other great sites and apps similar to Gravity Simulator are Universe Sandbox (Paid), Cosmographia (Paid, Open Source), Digital Universe Atlas (Free) and Mitaka (Free).
Celestia is an application for real-time 3D visualization of space, with a detailed model of the solar system, over 100,000 stars, more than 10,000 galaxies, and an extension mechanism for adding more objects.
The Digital Universe Atlas incorporates data from dozens of organizations worldwide to create the most complete and accurate 3-D atlas of the Universe from the local solar neighbourhood out to the edge of the observable Universe.
Mitaka is software for visualizing the known Universe with up-to-date observational data and theoretical models. Mitaka users can seamlessly navigate through space, from the Earth to the edges of the known Universe.