The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
26. And thus also (we must conclude, viz. that Brahman is the subject of the previous passage), because (thus only) the declaration as to the beings, &c. being the feet is possible.
That the previous passage has Brahman for its topic, we must assume for that reason also that the text designates the beings and so on as the feet of Gâyatrî. For the text at first speaks of the beings, the earth, the body, and the heart 2, and then goes on 'that Gâyatrî has four feet and is sixfold.' For of the mere metre, without any reference to Brahman, it would be impossible to say that the beings and so on are its feet. Moreover, if Brahman were not meant, there would be no room for the verse, 'Such is the greatness,' &c. For that verse clearly describes Brahman in its own nature; otherwise it would be impossible to represent the Gâyatrî as the Self of everything as is done in the words, 'One foot of it are all the beings; three feet of it are what is immortal in heaven.' The purusha-sûkta also (Rik
[paragraph continues] Samh. X, 90) exhibits the verse with sole reference to Brahman. Smriti likewise ascribes to Brahman a like nature, 'I stand supporting all this world by a single portion of myself' (Bha. Gîtâ X, 42). Our interpretation moreover enables us to take the passage, 'that Brahman indeed which,' &c. (III, 12, 7), in its primary sense, (i.e. to understand the word Brahman to denote nothing but Brahman.) And, moreover, the passage, 'these are the five men of Brahman' (III, 13, 6), is appropriate only if the former passage about the Gâyatrî is taken as referring to Brahman (for otherwise the 'Brahman' in 'men of Brahman' would not be connected with the previous topic). Hence Brahman is to be considered as the subject-matter of the previous passage also. And the decision that the same Brahman is referred to in the passage about the light where it is recognised (to be the same) from its connexion with heaven, remains unshaken.
95:2 Concerning the difficulty involved in this interpretation, cp. Deussen, p. 183, note.