With 2D Rigidbody is quite easy to do this, you can set the gravity scale to a negative value, but with 3D Rigidbody there is no option to change the gravity scale. One way to do this by adding a constant force component to the GameObject, and insert a value on the y-axis to be greater than the gravity (ex: 19.6 , from 9.8 multiply by 2). I'm not sure is this actually physically correct, but I've tested it, and it does the job...