Here is a complete mention of Gayatri Mantra in Padmapurana
In Padmapurana Brahma detailed each relevant Deity in respect of each of the Twenty Four Letters of the Gayatri Mantra as follows: Agni, Vayu, Surya, Viyat (Akash),Yamaraja, Varun, Brihaspati, Parjanya, Indra, Gandharva, Pusha, Mitra, Twashta, Vasu, Marudganas, Soma, Angira, Vishwa Deva, Ashvini Kumars, Pragapati, Sampurna Devata, Rudra, Brahma and Shri Vishnu. The letter-to-letter seriatum of Deities represented in the Gayatri Mantra thus provided the ‘Vangmaya’/ ‘Vani Vishaya’ or the cross-referencing of the Letter-Deity implication. As regards the ‘Nyasa’ of the Aksharas of the Gayatri Mantra versus Body Limbs, the explanation is as follows: ‘Tat’-touch the thumbs of the feet; ‘Sa’-Gulpha (ankles); ‘Vi’-the two pindaliyas (calves); ‘thu’-ghutnas (Knees); ‘Var’-jangha (lap); ‘Re’-Terminal Part; ‘Nya’- ‘Andakosha; ‘M’-‘Kati bhagas’( thighs); ‘Bha’-Nabhi Mandala; ‘orgo’-Udaram; ‘Dey’-both the breasts; ‘Va’- Hridaya; ‘Sya’-both the hands; ‘Dhee’-face; ‘Ma’-taalu; ‘hi’-top of Nasika( nose); ‘Dhi’-both the eyes; ‘Yo’- both bahus (hands); ‘Yo’- Lalaat; ‘Nah’-Muhka’s Purvabhaaga; ‘Pra’-‘Dakshna bhaag; ‘Cho’-Paschima bhaaga; ‘da’-Mukha’s Uttara Bhaga; ‘Yaat’-Mastak (head). By performing the Nyasa as above, all the ‘Aksharas’or the Letters are thus aligned to Brahma-Vishnu and Maheswara.
Another kind of Nyasa for the Sandhya-Kaalas is as follows:
Om Bhuriti Hridaye
Om Bhuvah Shirasi
Om Swaha Shikhayai Om
Bhargo Devasya Dhimahi Netrayoh Om
Dhiyoyonaha prachodayaatiti Karayoh
Om Apo jyotee rasomritam Brahma
The above Vyahruti and Twelve Omkaaraas along with Gayatri Mantra be performed at Sandhya Timings while touching water (Jala sparsha) followed by Pranayama Kriya thrice; then should follow the Gayatri Mantra Japa for as many times as possible. Then should follow ‘Punah-Nyasa’ or Repeat Nyasa as follows: Om Bhuh Padaabhyaam and touch the feet; Om Bhuvah Jaanubhyaam and touch both ‘ghutane’ (knees); Om Swahah Katyaam and touch kati bhaaga’ (thighs); Om Mahah Naabhou and touch Nabhistaan (navel); Om Janah Hridaye and touch Hridaya; Om Tapah Karaye and touch both the hands; Om Satyam and touch Lalaatey (Forehead) and finally recite the entire Mantra and touch the Shikha (tuft).