ऋचो यजूंषि सामाानीत्यष्टावक्षरा-ण्यष्टाक्षरँ ह वा एकं गायत्र्यै पदमेतदुहैवास्या एतत्स यावतीयं त्रयी विद्यातावद्ध जयति योऽस्या एतदेवं पदं वेद ॥
In plain words
Ṛk, Yajus, and Sāman — say the three together and you have eight syllables. That is one whole foot of the Gāyatrī, hers entirely. Whatever the three Vedas reach, that much belongs to the one who knows this foot of hers in just this way.
Close to the Sanskrit
The ṛc verses, the yajus formulas, and the sāman chants — thus these are eight syllables. Eight-syllabled indeed is the one foot of the Gāyatrī. This indeed is hers, this. As much as this threefold knowledge is, that much he conquers who thus knows this foot of hers.