What Rabban Gamliel Said!

(5) Rabban Gamliel would say: Anyone who did not say these three matters on Passover has not fulfilled his obligation: The Paschal lamb, matza, and bitter herbs. The Paschal lamb is brought because the Omnipresent passed over [pasaḥ] the houses of our forefathers in Egypt, as it is stated: “That you shall say: It is the sacrifice of the Lord’s Paschal offering for He passed over the houses of the children of Israel in Egypt, when he smote the Egyptians, and delivered our houses” (Exodus 12:27). The reason for matza is because our forefathers were redeemed from Egypt, as it is stated: “And they baked the dough that they took out of Egypt as cakes of matzot, for it was not leavened, as they were thrust out of Egypt and could not tarry, neither had they prepared for themselves any victual” (Exodus 12:39). The reason for bitter herbs is because the Egyptians embittered our forefathers’ lives in Egypt, as it is stated: “And they embittered their lives with hard service, in mortar and in brick; in all manner of service in the field, all the service that they made them serve was with rigor” (Exodus 1:14).

The attendants poured the second cup for the leader of the seder, and here the son asks his father the questions about the differences between Passover night and a regular night. And if the son does not have the intelligence to ask questions on his own, his father teaches him the questions. The mishna lists the questions: Why is this night different from all other nights? As on all other nights we eat leavened bread and matza as preferred; on this night all our bread is matza. As on all other nights we eat other vegetables; on this night we eat bitter herbs. When the Temple was standing one would ask: As on all other nights we eat either roasted, stewed, or cooked meat, but on this night all the meat is the roasted meat of the Paschal lamb. : As on all other nights we dip thr, on this night we dip twice.

And according to the intelligence and the ability of the son, his father teaches him about the Exodus. He begins with the Jewish people’s disgrace and concludes with their glory. And he expounds from the passage: “An Aramean tried to destroy my father” (Deuteronomy 26:5), until he concludes explaining the entire section.

MISHNAH: One mixes him the second cup, and here the son asks. If the son does not know how to ask, his father instructs him: What is the difference between this night and all other nights? For every night we dip once, but this night we dip twice. For every night we eat leavened and unleavened bread, but this night only unleavened. For in all other nights we eat meat roasted, preserved, or cooked, but in this night only roasted. According to the son’s understanding the father teaches him. He starts with ignominy and ends with praise and explains from “a lost Aramean was my father” until he finishes the paragraph.
Rabban Gamliel used to say, anybody who did not teach these three words on Passover did not fulfill his duty, Pesaḥ, mazzah, and bitter herbs

Rabban Gamliel used to say: . The other version which says, “Rabban Gamliel says…....
It is possible that Rabban Gamliel disagrees with Rabbis Yose, Eliezer, and Akiva. It is possible that they are stating that we are only obligated to tell the story of the Exodus (the enslavement of the Israelites and the ten plagues) and not the reason for the commandments. He follows their statements by saying, “That is not enough. If you haven’t explained the reason for pesah, matzah and maror, you still haven’t fulfilled the obligation of the night.” If this explanation is correct, then the proper reading of the text is, “Rabban Gamliel said….” Even so, those who fear God will fulfill both practices: Telling the story of the Exodus and explaining the different Seder practices. That is why we say Rabbis Akiva, Yose and Eliezer’s statements as well as Rabban Gamliel’s statement in our Seder

(8) They shall eat the Passover lamb that same night; they shall eat it roasted over the fire, with unleavened bread and with bitter stuff.

You need to say "this paschal offering that we are eating" and he connects Mazah and Maror with this to say so to with "this matzah" and "this Maror"

Explains their reasoning.

Rabbi Baruch Halevi Epstein (the Torah Temima) Rabban Gamliel used to say: Anyone who has not mentioned these three things on Passover has not fulfilled his obligation. Why does Rabban Gamliel single out these three Mitzvot as needing some type of explanation? In point of fact he was of the opinion that Mitzvot in general need “Kavanah,” “intention,” which means that if we can’t explain why we are doing them then we haven’t really fulfilled them properly (See Berachot 13a.) To act without some sense of purpose was not sufficient to fulfill the obligation. These three Mitzvot, however, are different. While other Mitzvot need “Kavanah,” these three Mitzvot demand both inner intention as well as some form of verbal expression of why we are performing them. That is why Rabban Gamliel says, “Anyone who has not mentioned...”

And that which you say, does not fulfil obligation, means not fulfil it to its fullest, seeing as we find The Torah is stringent on 'saying' in order we can recall the wonders of the creator as we remember in our mouths what they are bringing.