Bhashya
तथा प्राणः अपानः व्यानः एतान्यपि प्राणाद्यभिधाना-क्षराणि अष्टौ ; तच्च गायत्र्यास्तृतीयं पदम् ; यावदिदं प्राणि-जातम् , तावत् ह जयति, योऽस्या एतदेवं गायत्र्यास्तृतीयं पदं
In plain words
In the same way, prāṇa, apāna, and vyāna give eight syllables again, and that is the third foot of the Gāyatrī. Whatever breathes anywhere belongs to the one who knows this third foot in just this way — [the sentence is cut short in the source at exactly this point].
Close to the Sanskrit
Likewise, prāṇa, apāna, vyāna — these too, the syllables named prāṇa etc., are eight. And that is the third foot of the Gāyatrī. As much as this whole class of living, breathing beings is, that much he conquers who thus knows this third foot of the Gāyatrī — [the source text breaks off here, mid-sentence, before the expected closing verb 'knows' (veda)].