Gangetic means of the Ganges, or from the land and water around that river. The Ganges is the long river that flows through northern India and into Bangladesh, feeding the fields and drawing Hindu pilgrims who wash in it. Writers use Gangetic when they want the whole river-world, not the river’s own name: the Gangetic plain (a vast, crowded stretch of farmland), Gangetic dolphins, Gangetic soil. It is a geography and science word. You will meet it in textbooks, maps, and nature films far more than in everyday talk. It stays an adjective. You do not say “the Gangetic” to mean the river; the river is still the Ganges. Like other -ic place words (Pacific, Nile → Nilotic), it sounds a little learned, and that is honest: it belongs to careful description, not to small talk.