GUIText.fontStyle  

var fontStyle : FontStyle

Description

The font style to use (for dynamic fonts)

If this is set to a value other then normal, the font style set in the font importer is overriden with a custom style. This is only supported for fonts set to use dynamic font rendering. Other fonts will always render in normal style.

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