Hogwarts Legacy Gobstones are one of the many collectibles you're able to locate around the magical school, but in this case, you're gathering them for a first-year student called Zenobia Noke. After winning too much, her housemates hid her Gobstones around the school in out-of-the-way places so she couldn't get them back.
You can find Zenobia Noke after your first Charms and Defence Against the Dark Arts lessons—just look for the "Gobs of Gobstones" quest marker in the same area. With your newly learnt Accio spell...