player.gameObject.SendMessage("gravity", 50.0f);

using UnityEngine;
using System.Collections;

public class Material : MonoBehaviour {
   
  public GameObject go;
  public GameObject player;
  bool stone=false;
  bool paper=false;
  bool planar=true;
  public Texture[] texture;
  ThirdPersonController ControlScript;
  // Use this for initialization
  void Start () {
   ControlScript =  player.gameObject.GetComponent<ThirdPersonController>();
  }
   
  // Update is called once per frame
  void Update () {
  if (Input    .GetKeyDown    (KeyCode    .Z    ))
       {
    stone = true;
    paper = false;
    planar= false;
    go.renderer.material.mainTexture = texture[0];
   }
    if (Input    .GetKeyDown    (KeyCode    .X    ))
       {
    stone = false;
    paper = true;
    planar= false;
    go.renderer.material.mainTexture = texture[1];
   }
    if (Input    .GetKeyDown    (KeyCode    .C    ))
       {
    stone = false;
    paper = false;
    planar = true;
    go.renderer.material.mainTexture = texture[2];
   }
    
   if (stone)
   {
    ControlScript.gravity = 50.0f;
   }
  }
}