What people recite as the Gayatri and what stands in the Rig Veda are not quite the same length, and the difference is instructive about how liturgy works.
The verse proper is Rig Veda 3.62.10, twenty-four syllables in the metre that gives it its name, ascribed to the seer Vishvamitra. It asks that the desirable radiance of Savitr be meditated on, and that he impel our thoughts. Savitr is the impelling or stimulating aspect of the sun, not the visible disc, and the verb at the end is causative, the request is that the god set our minds in motion.
The recitation adds material at the front. The opening syllable and the three vyahritis, bhur, bhuvah, svah, naming earth, atmosphere and sky, come from the ritual manuals, not from the hymn. They are a liturgical frame, added when the verse was taken out of its hymn and used as a formula in its own right. Longer forms add four more of these worlds. None of that is in the Rig Veda.
Its date cannot be fixed. The family books of the Rig Veda are conventionally placed in the second millennium BCE, and that is a scholarly convention resting on linguistic comparison rather than on any datable event. Anyone giving a year is making it up.
The metre matters for the sound. Gayatri metre is three lines of eight syllables, and the received text of 3.62.10 in the Samhita is often reckoned to fall a syllable short, which the recitation tradition handles by restoring a vowel that had contracted.