guiText.fontStyle = FontStyle.BoldAndItalic;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
guiText.fontStyle = FontStyle.BoldAndItalic;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
guiText.fontStyle = FontStyle.BoldAndItalic