"Whoever sings is protected, " that is "Gayatri". "Gayantam trayate yasmat Gayatri' tyabhidhiyate."
"Sings" is not used here in the sense of singing a song. It means intoning or chanting (the mantra) with affection and devotion. People who chant the Gayatri in this manner are protected. While speaking about this mantra the Vedas use these words : "Gayatrim Chandasam mata". "Chandas" means the Vedas. So Gayatri is the mother of all Vedic mantras (that is the Vedas proclaim them so). It has twenty-four aksaras (letters or syllables) and three feet, each foot of eight syllables. That is why the mantra is called "Tripada Gayatri". Each foot is the essence of a Veda. Thus Gayatri is the essence of Rgveda, Yajurveda and Samaveda. The Atharvaveda has its own Gayatri. To receive instruction in it you must have a second upanayana.
Says the Manusmriti: "Tribhya eva tu Vedebyhah padam padamaduduham". It means that each pada of Gayatri is taken from one of the (three) Vedas. We have forsaken all else that is Vedic. What will be our fate if we give up the Gayatri mantra also?
Gayatri-japa is essential to all rites performed according to the sastras.
(see also chapter 12)
For a general background, please see here