When the tool is enabled, it shows the gravity specific object properties of the currently selected objects.
To create a simulation of two planet system, orbiting each other:
1. Create two analytic spheres representing the planets. The scale could be such that the radiuses of the spheres is 0.1 m and the distance between the spheres 1m.
2. Select the both spheres and enable the Gravity tool. Set Mode to 'Both', set 'Mass' to some very high value (say 10 000 000 kg). If the distance between the spheres is say 1 m, then this mass will generate noticeable gravity force.
3. Optionally, use the Velocity tool to define initial velocity for the spheres so that they will orbit around rather than just oscillate back and forth due to gravity.