var other : GameObject; 
other.guiText.text = "HelloWorld";

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public GameObject other;
    void Example() {
        other.guiText.text = "HelloWorld";
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public other as GameObject

    def Example():
        other.guiText.text = 'HelloWorld'