Why I can't use GUI.* in unity2D?

Why I can’t use all GUI.* function ( or what are them ) while I’m using Unity2D?
Excist a method to enable this “option”?
In every C# Script, not in a specific file.

Don’t name your classes the same thing as Unity classes, since that confuses Unity unless you explicitly differentiate between them. Nothing to do with 2D. (There is no actual Unity2D, by the way…that’s just a toggle in the scene view mode that affects the scene camera and nothing else; Unity works exactly the same otherwise. Well, that and the default texture import type.)