In order to predict/calculate object data (position and velocity) in future or past time (within limits of calculation) one need object mass, initial position and initial velocity vector at some initial moment of time.
Gravity Nu has 3 components; Gravity Nu main component (player), Project Editor and Impulse Editor.
It is mostly about numerical parameters - all other stuff (textures, themes, etc.) is for better understanding via visualization or just for fun. Although you can see planets and comets in Solar System this is the only free software (we are aware of) in which you can design and track your own objects (for instance, spaceship launched from Earth at some particular moment in time).
Once the initial object mass, position and velocity is defined you can see the behavior of system in time (forward or backward). You can create the system consisting of light (1 kg object) or heavy (planet, star) objects and see what happens in time.
JPL HORIZONS data (EPHEMERIS) can be used if you want to design (copy of) space system consisting of actual objects (planets, satellites etc.). If you use JPL HORIZONS data all objects should share same 'real' initial moment in time (date & time).
You can even design your impulse programmed object (spaceship) which trajectory is changed at some moments in time with jet propulsion impulse applied (the only present way to change trajectory after launch).
