Jerusalem was in turmoil in the period prior to its destruction (70 ACE). The Romans had it under siege. The Jews themselves were divided into multiple factions and sects - some held to elite establishment of the Priests (Sadducees), others sided with the rabbinic scholars (Pharisees) who tried to popularize Judaism and develop a code of Jewish law that governed all aspects of Jewishlife. The Sicarii (dagger men) assassinated anyone they perceived as a Roman sympathizer and gave rise to the Zealots who ultimately occupied Masada. Legend has it that Rabbi Yochanan ben Zakkai, a leading Rabbinic scholar, was smuggled out of Jerusalem in a coffin in order that he might negotiate a pardon for his followers. Vespasian permitted him to travel to the town of Yavne and establish a new center for rabbinical Judaism. When he passed, Rabban Gamliel II assumed the role of the President (Nasi) of the Sanhedrin (Jewish Supreme Court). Although a somewhat controversial leader who was involved in many a fracas with his Rabbinic colleagues, he did much to restore and reinvigorate Judaism after the destruction of the Temple.
Rabat Gamliel's statement about what must be recited on Passover is actually the answer to the original 3 questions of the Haggadah (before they became 4 questions!) The original 3 questions were 1. Why do we eat Matzah? 2. Why do we dip into maror? 3. Why do we eat only roasted meat? Over the generations the 3 questions were modified and a 4th question was added. In addition, the Questions were separated from Rabban Gamliel's Answers!

haggadah Section: -- Cup #2 & Dayenu