Gyroscope.gravity

Description

Returns the gravity acceleration vector expressed in the device's reference frame.

var movementScale: float;

function Update() {
    // A "spirit level" - the dot product of the gravity and the Y axis (ie, Vector3.up)
    // is a measure of how far the device is from level on that axis (it will be zero
    // if the device is perfectly level). This value can be used to position an object
    // to act as the "bubble".
    var pos = transform.position;
    pos.y = Vector3.Dot(Input.gyro.gravity, Vector3.up) * movementScale;
    transform.position = pos;
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public float movementScale;
    void Update() {
        Vector3 pos = transform.position;
        pos.y = Vector3.Dot(Input.gyro.gravity, Vector3.up) * movementScale;
        transform.position = pos;
    }
}