Short answers, pulled from the story.
Jus gentium, Latin for "law of nations" or "law of peoples," was the body of customary law in Roman legal tradition thought to be held in common by all peoples. It was not a statutory code but a framework derived from natural reason, applying to all persons regardless of Roman citizenship. The jurist Gaius defined it as what "natural reason has established among all peoples."
Ius civile was the body of law specific to Roman citizens, while jus gentium applied to all people, including non-Romans. Jus gentium was grounded in natural reason thought to be innate in all of mankind, whereas ius civile depended on the particular statutes and customs of the Roman state.
Ulpian, a 2nd-century Roman jurist, divided law into three branches: natural law, the law of nations, and civil law. He noted that under natural law all people are born free, but slavery was supported by the jus gentium. This meant the law of nations could diverge from natural law on a fundamental point.
Hermogenianus, writing in the second half of the 3rd century, described jus gentium as covering wars, national interests, kingship and sovereignty, rights of ownership, property boundaries, settlements, and commerce, including contracts of buying, selling, letting, and hiring.
By the 16th century, the unified concept of jus gentium fragmented under three pressures: individual European nations developed distinct bodies of law, the authority of the Pope declined, and colonialism created subject nations outside the West that had no recognized standing in the existing framework.
In his work The Law of Peoples, John Rawls stated that his concept of the law of peoples is drawn from the traditional jus gentium. He made specific reference to the phrase ius gentium intra se, meaning "the law of peoples within themselves."