FontStyle.Bold  

guiText.fontStyle = FontStyle.Bold;

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        guiText.fontStyle = FontStyle.Bold