How do graduate schools evaluate GPA?

I am wondering how graduate schools like Stanford, Berkeley, Caltech, and MIT evaluate the GPA?

Because you can boost your GPA by taking easy classes and in the application it only says enter your GPA. Do they consider classes students have taken? Do they consider whether course are challenging or not?

Answer

As a filter.

The main purpose of asking for GPA in grad school admissions is to quickly weed out people (like me) who have actually low GPAs. Nobody is admitted to top PhD programs solely on the basis of their GPA, or even primarily on the basis of their GPA. Other factors, including which classes you took, are more important.

If you have a high GPA, but no evidence of research potential, you will not be admitted.