The Gathas are the oldest layer of the Avesta and the only part attributed to Zarathushtra himself. There are seventeen of them, arranged in five groups by metre, and the Ahunavaiti is the first and longest group.
What is striking about them is the voice. They are not liturgy in the ordinary sense. They are a man arguing: pressing questions on Ahura Mazda directly, complaining that he has no following and few cattle, naming his opponents, and asking what reward the righteous can expect. There is a passage in which he asks to which land he should flee, since he is rejected by his own. Another asks a series of questions about the natural order, who fixed the path of the sun and stars, who holds up the earth, who made light and darkness, in a form that anticipates the divine speeches in Job by a long way.
The central opposition is asha, truth or right order, against druj, the lie, and the human situation is a choice between them. The choice is explicitly free, which is the point on which most later Zoroastrian theology stands.
The language is Gathic Avestan, an archaic dialect close enough to the Sanskrit of the Rig Veda that the two can be mechanically converted in places, a foundation of comparative Indo-Iranian philology and the strongest argument for an early date, since a language this near to Vedic sits awkwardly with the sixth century BCE that traditional Zoroastrian chronology assigns its speaker.
Their liturgical position is central. They stand at the heart of the Yasna, the principal Zoroastrian act of worship, surrounded by later Avestan material that frames them, so that the oldest layer of the tradition is recited daily inside a structure built around it centuries afterward.