Sri Gayatri Mantra

Gayatri Mantra is considered as mother of veda. The mantra is the foremost revered mantra in Hinduism. It inspires wisdom. For the first time it got recorded in Rig Veda (Reference: iii, 62, 10). The Veda was written in Sanskrit about 2500 to 3500 years ago so also the GayatrMantra. However this moola mantra may have been chanted for many generations before that.
The word Gayatri is formed from combination two important Sanskrit words:
1. gayanath = what is sung, giving of praise (or) gaya = vital energies
2. trayate = preserves, protects, gives deliverance, grants liberation
3. mantra = an instrument of thought, a sacred text. a prayer of praise
Now you will understand the overall meaning of Gayatri Mantra as: a prayer of praise that awakens the vital energies and gives liberation.

Sri Gayatri

Om Bhur Bhuvah Suvaha
Tat Savitur Varenyam
Bhargo Devasya Dheemahi
Dhiyo Yo Nah Prachodayat

Meaning

"May the Almighty God illuminate our intellect to lead us along the righteous path". The mantra is also a prayer to the "giver of light and life" - the sun.

Oh God! Thou art the Giver of Life,

Remover of pain and sorrow,

The Bestower of happiness,

Oh! Creator of the Universe,

May we receive thy supreme sin-destroying light,

May Thou guide our intellect in the right direction

There are many more versions of the Gayatri Mantra in which deities other than Savitur (sun) are invoked.
To quote example, there is Agni Gayatri, Durga Gayatri, Shiv Gayatri, Sri Shanmuga Gayatri, Vishnu Gayatri and Shiva Gyatri, On the whole, these Gayatri mantras are more or less similar, and we may consider them as derivation of the Gayatri of your Rig Veda.
Related Post: Gayatri on other Deities
Ganesha Gayatri
Sri Shanmuga Gayatri