GUIContent.text  

function OnGUI () {
    GUI.Button (Rect (0, 30, 100, 20), GUIContent ("Click Me"));
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnGUI() {
        GUI.Button(new Rect(0, 30, 100, 20), new GUIContent("Click Me"));
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        GUI.Button(Rect(0, 30, 100, 20), GUIContent('Click Me'))