Like Aditya hridayam, I came across Gayatri hridayam. What exactly are hridyam mantras? What is gyatri hridayam and why is it used to pray to Godess gyatri before doing the sandhya gyatri?.
The Gayatri Hridayam is a mantra found in this chapter of the Devi Bhagavata. The chapter claims that the mantra is "explicitly written in the Atharva Veda", but you can read the Atharvana Veda here; I don't see it. In any case, here is the preamble to the mantra:
The Risi of this Gâyatrî Hridaya is Nârâyana; the Chhandas is Gâyatrî; and S’rî Parames'varî Gâyatrî is the Devatâ.
That's a reference to the sage Narayana, an ancient incarnation of Vishnu who was the brother of the sage Nara and the founder of Pancharatra, as I discuss here. (Nara and Narayana were the previous births of Arjuna and Krishna respectfully.) In any case, here are the Dhyana shlokas (which I discuss here) of the Gayatri Hridayam:
First, consider the Gâyatrî, the Devî, the Mother of the Vedas as of a Cosmic Form (Virâ trupâ) and meditate all the Devas as residing on Her Body. Now in as much as the Pinda and Brahmânda are similar, consider yourself as of the form of the Devî and meditate within yourself on the Devatâs[.]
Then a Nyasam is prescribed, after which the actual hymn is given:
Thinking thus, repeat “I take refuge of the Divine Holy Gâyatrî, the Chief and most Excellent One, the Thousand eyed and I take refuge wholly unto Her.” Then repeat “I bow down to Tat savitur varenyam,” “I bow down to the Rising Sun on the East,” “I bow down to the Morning Aditya,” “I bow down to the Gâyatrî, residing in the Morning Sun” and “I bow down to all.”
And here are the benefits described:
Whoever recites this Gâyatrî Hridaya in the morning finds all the sins committed in the night all destroyed! Whoever recites this in the evening gets his sins of the day all destroyed! Whoever recites this in the evening and in the morning can rest assured to have become free of sins; he gets the fruits of all the Tîrthas; he is acquainted with all the Devas; he is saved if he has spoken anything that ought not to have been spoken; if he has eaten anything that is not fit to be eaten; if he has chewn and sucked anything that ought not to have been chewn and sucked; if he has done anything that ought not to have been done and if he has accepted hundreds and thousands of gifts that ought never to have been accepted.
The sins incurred by eating with the others in a line cannot touch him. If he speaks lies, he will not be touched by the sins thereof; even if a non-Brahmachâri recites this, he will become a Brahmachârî. O Nârada! What more shall I say to you of the results of Gâyatrî Hridaya than this :-- that whoever will study this will acquire the fruits of performing thousand sacrifices and repeating the Gâyatrî sixty thousand times. In fact, he will get Siddhi by this. The Brâhmâna, who daily reads this in the morning will be freed of all the sins and go upwards to the Brahmâ (Loka) and is glorified there. This has been uttered by Bhagavân Nârâyana Himself.
One note of caution: the Devi Bhagavata Purana is considered to have a lot of interpolations, and is considered by some not to even be a genuine Upapurana, so take this with a grain of salt.
As to why it's called the Gayatri Hridayam, Hridaya meaning heart, the only explanation I've comes across is the one given by Vaishnavas in explaining why the Aditya Hridayam is included in the Ramayana. The explanation is that the Aditya Hridayam is not addressed to Surya the sun god, but rather to the being who lies at the Hridaya or heart of Surya, namely Vishnu, since he's the Paramatma or supreme soul and thus the Antaryami of all the gods. So by that argument, the Gayatri Hridayam would also be a mantra to Vishnu.
By the way, the Gayatri Hridayam shouldn't be confused, with the Gayatri Panajara, another hymn to Gayatri which I discuss here.