In Vedic Sanskrit poetry, a chandas (छन्दस्) is a poetic meter that determines the rhythmic structure of a verse.
Gāyatrī is one such meter, it traditionally characterized by three pādas (metrical lines), each containing eight syllables, for a total of 24 syllables.
Tat savitur vareṇyaṃ
Bhargo devasya dhīmahi.
Dhiyo yo naḥ pracodayāt.
As you can see, this is a combination of 8 x 3 = 24
So any verse or mantra that follows this meter can be called a “Gayatri Mantra”.
This was the mantra in its original form.
Another common misconception is that Sage Vishwamitra originally composed the now popular Gayatri Mantra. But as we are aware, the Vedas are Apaurusheya and were not composed by anyone.
Only the Mahavyahritis or the perfunctory formula “bhūr bhuvaḥ svaḥ” was revealed to the sage while the rest of the mantra pre-dated him.
The addition of the Omkara or Om is then prescribed in the Taittirīya Āraṇyaka illustrating how the components are joined in Vedic ritual tradition.
Another example, the first verse of the Rig Veda Itself is also a Gayatri Mantra:
Agnim īḷe purohitaṃ (8 syllables*)*
Yajñasya devam ṛtvijam, (8 syllables)
Hotāraṃ ratnadhātamam. (8 syllables)
There are 1000s of Gayatri Mantras in the Vedic corpus. So it is not accurate to claim that there is one Gayatri Mantra and that it is dedicated to a Goddess named Gayatri. This is a much later theological development.
So who exactly is Gayatri Mantra dedicated to?
In the Vaishnava traditions, it is a mantra dedicated to none other than Lord Narayana himself.
The mantra invokes Savitṛ or the solar orb which can be understood as the locus in which the presence of the supreme lord is contemplated.
Dhyeyaḥ sadā savitṛ-maṇḍala-madhyavartī
Nārāyaṇaḥ sarasijāsana-sanniviṣṭaḥ |
Keyūravān makara-kuṇḍalavān kirīṭī
Hārī hiraṇmaya-vapur dhṛta-śaṅkha-cakraḥ ||
“One should always meditate upon Nārāyaṇa, who resides in the centre of the solar orb, seated upon a lotus, adorned with armlets, wearing makara-shaped earrings and a crown, decorated with a necklace, possessing a golden-hued body, and bearing the conch and discus."
Hare Krishna!