I believe the significance of the conformity/rebellion theme in Fitzgerald's The Great Gatsby has to do with Fitzgerald urging his readers the dangers of both. Rarely does Fitzgerald come down soundly on one side of an issue or another. He points out the reality of life, without judgement, simply the results.
We can see these two characteristics manifested in various characters within the novel. Jordan Baker is a great example of rebellion. Jordan flew in the face of all things that were supposed to be feminine, and therefore "right" for her. She choose to go against all those societal norms.
Nick, the narrator, could be considered an example of conformity. He simply wanted to be like all the people around him--he just wanted to fit in. So he found himself going to all the same parties as Gatsby and observing and trying to be involved in Gatsby's life.
This would have been a very important theme in the late 20s and 30s, as the nation had ERUPTED after the depression, making up for lost time and enjoying everything they could. Fitzgerald paints a picture of the excessive side of this time period, and the consequences thereof, whether you tried to conform, or you tried to rebel.