Bhashya
ॐ प्राणो गायत्रीत्युक्तम् ।कस्मात्पुनः कारणात् प्राण-भावः गायत्र्याः, न पुनर्वागादिभाव इति, यस्मात् ज्येष्ठश्चश्रेष्ठश्च प्राणः, न वागादयो ज्येष्ठ्यश्रैष्ठ्यभाजः ; कथं ज्ये-ष्ठत्वं श्रेष्ठत्वं च प्राणस्येति तन्निर्दिधारयिषया इदमारभ्यते ।अथवा उक्थयजुःसामक्षत्रादिभावैः प्राणस्यैव उपासनमभि-हितम्, सत्स्वपि अन्येषु चक्षुरादिषु ; तत्र हेतुमात्रमिहआनन्तर्येण संबध्यते ; न पुनः पूर्वशेषता ।विवक्षितं तुखिलत्वादस्य काण्डस्य पूर्वत्र यदनुक्तं विशिष्टफलं प्रा-णविषयमुपासनं तद्वक्तव्यमिति ।यः कश्चित्, ह वै इत्य-वधारणार्थौ ; यो ज्येष्ठश्रेष्ठगुणं वक्ष्यमाणं यो वेद असौभवत्येव ज्येष्ठश्च श्रेष्ठश्च ; एवं फलेन प्रलोभितः सन्प्रश्नाय अभिमुखीभूतः ; तस्मै चाह— प्राणो वै ज्येष्ठश्च
In plain words
Om. We were told that Prāṇa is Gāyatrī. So why say that of Prāṇa, and not of speech or any of the other faculties? Because it is Prāṇa that is truly eldest and best — speech and the rest simply don't carry that rank. But how does eldership and bestness belong to Prāṇa? It's to settle exactly that question that this passage begins. Or, put another way: meditation on Prāṇa alone has already been taught through the forms of Uktha, Yajus, Sāman, and royal power, even though other things like the eye exist too — here, the point being made is simply the reason for that, following straight on from what came before, not tacked on as leftover material. What is really meant is this: this book has a gap in it, and a specific reward for meditating on Prāṇa was left unsaid earlier — that is what is about to be told now. "Whoever" means anyone at all; "indeed" and "verily" are just there for emphasis. Whoever comes to know this quality of being eldest and best, about to be explained, becomes exactly that — the eldest and the best. Tempted now by that reward, the student leans in, ready for the question. And to him the teacher says: "Prāṇa is indeed the eldest and..."
Close to the Sanskrit
Om. It has been said that Prāṇa is Gāyatrī. Now, for what reason is the status of Gāyatrī [predicated] of Prāṇa, and not rather the status of Speech and the other [faculties]? Because Prāṇa is the eldest and the best, [while] Speech and the rest do not share in eldership and bestness. How is it that eldership and bestness belong to Prāṇa? — it is with a desire to determine this that this [section] is begun. Or else: meditation on Prāṇa alone has been enjoined through the forms of Uktha, Yajus, Sāman, Kṣatra and so on, even though other things such as the eye and the rest also exist; there, here [in this section], only the reason [for that identification] is connected, in immediate sequence — not, however, as a mere supplement to what precedes. Rather, what is intended [to be conveyed] is this: because this section has a gap, a specific result of meditation concerning Prāṇa that was left unstated earlier is what is [now] to be told. 'Whoever' means anyone at all; 'ha' and 'vai' serve merely the purpose of emphasis; whoever knows the quality of being eldest and best that is about to be described, he indeed becomes the eldest and the best; thus, drawn in by the promised fruit, [the inquirer] becomes receptive to the question; to him he [the teacher] says — 'Prāṇa, verily, is the eldest and...'