Gravity

Settings → Gravity

Ribbon menu → Forces tab → Special Forces → Gravity Tool

Build → Forces → Gravity Tool
 
You can specify the magnitude and direction of the acceleration of gravity. For each part with mass, the gravitational force produces a point force at its center of mass.
 
For the option:
Do the following:
X
Enter the acceleration value in the x direction or select + or - to enter a standard positive or negative value.
Y
Enter the acceleration value in the y direction or select + or - to enter a standard positive or negative value.
Z
Enter the acceleration value in the z direction or select + or - to enter a standard positive or negative value.