Bhashya
गायत्र्या मुखविधानाय अर्थवाद उच्यते— एतद् ह किल वै स्मर्यते, तत् तत्र गायत्रीविज्ञानविषये; जनको वैदेहः, बुडिलो नामतः, अश्वतराश्वस्यापत्यम् आश्वतराश्विः, तं किल उक्तवान्; यत् नु इति वितर्के, हो अहो इत्येतत्, तत् यत् त्वं गायत्रीविद्ब्रूथाः, गायत्रीविदमस्मीति यदब्रूथाः, किमिदं तस्य वचसोऽनुरूपम्; अथ कथम्, यदि गायत्रीविन्, प्रतिग्रहदोषेण हस्तीभूतो वहसीति ।स प्रत्याह राज्ञा स्मारितः— मुखं गायत्र्याः हि यस्मात् अस्याः, हे सम्राट्, न विदांचकार न विज्ञातवानस्मि- इति होवाच; एकाङ्गविकलत्वात् गायत्रीविज्ञानं मम अफलं जातम् ।शृणु तर्हि; तस्या गायत्र्या अग्निरेव मुखम्; यदि ह वै अपि बह्विवेन्धनम् अग्नावभ्यादधति लौकिकाः, सर्वमेव तत्संदहत्येवेन्धनम् अग्निः—एवं हैवंविद् गायत्र्या अग्निर्मुखमित्येवं वेत्तीत्येवंवित् स्यात् स्वयं गायत्र्यात्मा अग्निमुखः सन् ।यद्यपि बह्विव पापं कुरुते प्रतिग्रहादिदोषम्, तत्सर्वं पापजातं संप्साय भक्षयित्वा शुद्धः अग्निवत् पूतश्च तस्मात्प्रतिग्रहदोषात् गायत्र्यात्मा अजरोऽमृतश्च संभवति ॥
In plain words
To explain what the Gāyatrī's mouth is, tradition remembers a story. King Janaka of Videha once confronted Budila, a descendant of Aśvatara, with a challenge: 'You told me you know the Gāyatrī. Then why do you walk around bloated like an elephant with everything you've taken as gifts? That's not what a real knower looks like.' Reminded by the king, Budila admitted it: 'Your Majesty, I never learned her mouth. That's the one piece I was missing, and it made my whole knowledge useless.' 'Then listen,' said Janaka. 'Fire is the Gāyatrī's mouth. Pile as much fuel as you like onto a fire, and it burns all of it away completely. In the same way, someone who truly knows that fire is the Gāyatrī's mouth becomes, in himself, the Gāyatrī's own Self, with fire as his own mouth. So even if he has taken on a mountain of wrongdoing — every improper gift, every fault that comes with it — he burns through all of it, the way fire does, and comes out pure. That very fault of wrongful gifts becomes, for him, the doorway to being ageless and immortal.'
Close to the Sanskrit
For the sake of describing the Gāyatrī's mouth, an explanatory account is told: this indeed, it is said, is remembered — there, regarding the matter of knowledge of the Gāyatrī: Janaka, king of Videha, [addressed] Budila by name, descendant of Aśvatara-Aśva, [known as] Āśvatarāśvi; he said to him — ['nu' here expresses doubt; 'ho' is the same as 'oh'] — 'You claimed to be a knower of the Gāyatrī, [you said] "I am a knower of the Gāyatrī" — is this in keeping with your own words? How is it, if you are a knower of the Gāyatrī, that you go about bloated like an elephant with the fault of accepting gifts?' He, reminded by the king, replied — 'Because, O Emperor, I did not know the mouth of this Gāyatrī, I have not understood it' — thus he said. 'Because of lacking this one limb, my knowledge of the Gāyatrī has become fruitless.' 'Listen, then,' [said Janaka]: 'Agni indeed is the mouth of this Gāyatrī. If ordinary people heap even a great deal of fuel onto a fire, the fire burns up all of that fuel entirely — likewise, one who knows thus, that Agni is the mouth of the Gāyatrī, knowing it in just this way, becomes such a knower himself, having the Gāyatrī as his Self, having Agni as his mouth. Even if he commits much wrongdoing — the fault of accepting improper gifts and the like — having consumed and devoured that entire mass of sin, he becomes pure, like fire, and cleansed; and from that fault of accepting gifts, having the Gāyatrī as his Self, he becomes ageless and immortal.'