Pesach, Matzah, and Maror - What Rabban Gamliel Really Meant

When Rabban Gamliel says you don't fulfill your obligation unless you say Pesach, Matzah, and Maror, the Rambam says it means "כראוי" - the proper way.

What obligation do you not fulfill without saying these three paragraphs? סיפור יציאת מצרים or the mitzvot of Pesach, matzah and maror? According to the 2nd opinion you need to understand these 3 mitzvot before you perform them, otherwise you haven't done them properly.

What is the נפקא מינה?