Gaudium et Spes

The author of a papal document is the name of the pope but do not use the title "Pope" before the name. 

An example of a print version of Gaudium et Spes:

Paul VI. (1965). Pastoral Constitution on the Church in the modern world: Gaudium et spes. Boston, MA: Pauline Books & Media

An example of an online version of Gaudium et Spes:

Vatican Council. (2005). Pastoral constitution on the Church in the modern world: Gaudium et spes, December 7, 1965. Vatican City: Vatican Archives.

In-text references would look like this:

(Paul VI, 1965)

Print Copy of Gaudium et Spes:

Pastoral Constitution On The Church In The Modern World — Gaudium et Spes. Vatican City: Libereria Editrice Vaticana, 1965. Print.

Pastoral Constitution on the Church in the Modern World - Gaudium et Spes. Washington: United States Catholic Conference, 1965. Print.

Online Copy of Gaudium et Spes:

Pastoral Constitution on the Church in the Modern World: Gaudium Et Spes, December 7, 1965. Vatican City: Vatican Archives, 2005. Internet resource. Date of access.

In-text citation: 

Well known texts can be cited simply by customary Latin title (or the first few words), followed by what paragraph is being used.

(Gaudium et Spes par. 4)