Physics.gravity

Description

The gravity applied to all rigid bodies in the scene.

Gravity can be turned off for an individual rigidbody using its useGravity property.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void Example()
    {
        Physics.gravity = new Vector3(0, -1.0F, 0);
    }
}

ParticleSystem.gravityModifiers is Obsolete.