The Gayatri's placement in the family books, the oldest stratum of the Rig Veda, is secure on internal linguistic grounds. The absolute date of that stratum is not: proposed dates range across the whole second millennium BCE, and no inscription, datable artefact or external synchronism fixes any point in that range.
What would resolve this A securely dated inscription, artefact or archaeological correlate that can be tied to the language or content of the family books, comparable to the role the Mitanni treaty plays in dating other early Indo-Iranian material, would anchor the range.
OpenVedic philology, Indo-Iranian archaeologyStephanie W. Jamison and Joel P. Brereton, The Rigveda: The Earliest Religious Poetry of India