GravityScalar.cs

using UnityEngine;
using System.Collections;

[AddComponentMenu("Liberi/Gravity Scalar")]
[RequireComponent(typeof(Rigidbody))]
[Script(ScriptRole.Logic)]
public class GravityScalar : MonoBehaviour
{   
    public float GravityScale = 1f;

    void FixedUpdate ()
    {
        if (rigidbody.isKinematic)
            return;

        rigidbody.AddForce(Physics.gravity * (GravityScale - 1), ForceMode.Acceleration);
    }
}