The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
25. If it be objected that(Brahman is) not (denoted) on account of the metre being denoted; (we reply) not so, because thus (i.e. by means of the metre) the direction of the mind (on Brahman) is declared; for thus it is seen (in other passages also).
We now address ourselves to the refutation of the assertion (made in the pûrvapaksha of the preceding Sûtra) that in the previous passage also Brahman is not referred to, because in the sentence, 'Gâyatrî is everything whatsoever here exists,' the metre called Gâyatrî is spoken of.--How (we ask the pûrvapakshin) can it be maintained that, on account of the metre being spoken of, Brahman is not denoted, while yet the mantra 'such is the greatness of it,' &c., clearly sets forth Brahman with its four quarters?--You are mistaken (the pûrvapakshin replies). The sentence, 'Gâyatrî is everything,' starts the discussion of Gâyatrî. The same Gâyatrî is thereupon described under the various forms of all beings, earth, body, heart, speech, breath; to which there refers also the verse, 'that Gâyatrî
has four feet and is sixfold.' After that we meet with the mantra, 'Such is the greatness of it.' &c. How then, we ask, should this mantra, which evidently is quoted with reference to the Gâyatrî (metre) as described in the preceding clauses, all at once denote Brahman with its four quarters? Since therefore the metre Gâyatrî is the subject-matter of the entire chapter, the term 'Brahman' which occurs in a subsequent passage ('the Brahman which has thus been described') must also denote the metre. This is analogous to a previous passage (Kh, Up. III, 11, 3, 'He who thus knows this Brahma-upanishad'), where the word Brahma-upanishad is explained to mean Veda-upanishad. As therefore the preceding passage refers (not to Brahman, but) to the Gâyatrî metre, Brahman does not constitute the topic of the entire section.
This argumentation, we reply, proves nothing against our position. 'Because thus direction of the mind is declared,' i. e. because the Brahmana passage, 'Gâyatrî indeed is all this,' intimates that by means of the metre Gâyatrî the mind is to be directed on Brahman which is connected with that metre. Of the metre Gâyatrî, which is nothing but a certain special combination of syllables, it could not possibly be said that it is the Self of everything. We therefore have to understand the passage as declaring that Brahman, which, as the cause of the world, is connected with that product also whose name is Gâyatrî, is 'all this;' in accordance with that other passage which directly says, 'All this indeed is Brahman' (Kh. Up. III, 14, 1). That the effect is in reality not different from the cause, we shall prove later on, under Sûtra II, 1, 14. Devout meditation on Brahman under the form of certain effects (of Brahman) is seen to be mentioned in other passages also, so, for instance, Ait. Âr. III, 2, 3, 12, 'For the Bahvrikas consider him in the great hymn, the Adhvaryus in the sacrificial fire, the Khandogas in the Mahâvrata ceremony.' Although, therefore, the previous passage speaks of the metre, Brahman is what is meant, and the same Brahman is again referred to in the passage about the light, whose purport it is to enjoin another form of devout meditation.
Another commentator 1 is of opinion that the term Gâyatrî (does not denote Brahman in so far as viewed under the form of Gâyatrî, but) directly denotes Brahman, on account of the equality of number; for just as the Gâyatrî metre has four feet consisting of six syllables each, so Brahman also has four feet, (i. e. quarters.) Similarly we see that in other passages also the names of metres are used to denote other things which resemble those metres in certain numerical relations; cp. for instance, Kh. Up. IV, 3, 8, where it is said at first, 'Now these five and the other five make ten and that is the Krita,' and after that 'these are again the Virâg which eats the food.' If we adopt this interpretation, Brahman only is spoken of, and the metre is not referred to at all. In any case Brahman is the subject with which the previous passage is concerned.
95:1 The vrittikâra according to Go. Ân. in his tîkâ on the bhâshya to the next Sûtra.