Final answer:
Gretchen is watching the Parade of Nations, a staple of the Olympic opening ceremonies where athletes march into the stadium by country, symbolizing unity and national pride.
Step-by-step explanation:
The part of the Olympic Games that Gretchen is watching, where large groups of athletes are marching into the stadium separated by country and waving to the audience, is known as the Parade of Nations.
This event is a hallmark of the Olympic opening ceremonies, where athletes from around the world march in a procession that celebrates national identity and pride. Historically, it has been an opportunity for host countries to showcase their culture and for participating nations to carry their flags in a display of unity and diversity.
The Parade of Nations is different from other parts of the opening ceremonies, such as the lighting of the Olympic flame, which commemorates the ancient origins of the games and symbolizes the Olympic spirit. Cultural and artistic programs also form part of the festivities, but they are separate from the athletes' march, which is specifically meant to introduce the competitors to the audience.