The word "Gayatri" itself explains the reason for the existence of this mantra. It has its origin in the Sskrit phrase Gayantam Triyate iti, and refers to that mantra which rescues the chanter from all adverse situations that may lead to mortality.
Goddess Gayatri is also called "Veda-Mata" or the Mother of the Vedas - Rig, Yajur, Saam and Atharva - because it is the very basis of the Vedas. It is the basis, the reality behind the experienced and the cognized universe.
The Gayatri mantra is composed of a metre consisting of 24 syllables - generally arranged in a triplet of eight syllables each. Therefore, this particular meter (tripadhi) is also known as the Gayatri Meter or "Gayatri Chhanda."