Question: Why is Gayatri represented as a deity with fivefaces?

Ans:

Descriptions of deities and characters in mythology showing many heads and arms are common and may appear odd and paganish to a person not familiar with the subtleties of Indian spiritual tradition. Brahma and Vishnu have been described as having four faces, Shiva with five, Kartikeya with six, Durga with eight and Ganesha with ten heads. It is said that the demon king Ravana had ten heads and twenty arms; and sahastrabahu, another demon had a thousand hands. Here, the numbers do not refer to the physiology, but to characteristics of the divine or evil attributes of the deities or demons as the case may be. 

Indian spirituality frequently mentions five-fold classifications - such as the five basic elements of the cosmos (Tatvas); the five sheaths (Koshas) covering the human soul ; the five organs each of perception and action in the human body (Gyanendriyas and Karmendriyas), the five life -forces (Prans); the five types of energies operating in human bodies (Agnis); the five types of Yoga ....etc. The Gayatri Mantra , too is divisible in five parts namely (1) Om (2) Bhurbhuwaha Swaha (3) Tatsaviturvareniyam (4) Bhargo Devasya Dheemahi (5) Dhiyo Yonaha Prachodayat. Each of these corresponds to the five primary emanation of the supreme spirit: Ganesh, Bhawani, Brahma, Vishnu and Mahesh respectively. The entire super-science of spirituality too is encapsuled in the four Veda s and one Yagya . 

The five faces of Gayatri refer to these Divine attributes, which the Sadhak has to deal with in course of Sadhana