This scene comes in the first chapter, during Nick's visit with Tom and Daisy, and his first meeting with Jordan Baker. Nick and Daisy return from a personal talk to find Jordan reading to Tom from the newspaper. In an earlier scene, Nick remarks to the reader on Jordan's silence and stillness, and on the seemingly absolute control she has over her body. This comes from her profession; Jordan is a professional tennis player, and her body is her livelihood. She needs to have control over it to win, and she exhibits that control by not making unnecessary movements.
When we came in she held us silent for a moment with a lifted hand.
"To be continued," she said, tossing the magazine on the table, "in our very next issue."
Her body asserted itself with a restless movement of her knee, and she stood up.
"Ten o'clock," she remarked, apparently finding the time on the ceiling. "Time for this good girl to go to bed."
(Fitzgerald, The Great Gatsby, gutenberg.net.au)
It is likely that Jordan's strict control can slip when she is tired; she is shown yawning earlier and the "restless movement" is a typical body spasm. The body spasm is not Jordan asserting herself, as she does not will it intentionally; instead, her body expresses its general exhaustion through the spasm. It also shows, indirectly, that she is in control of the room and the situation, although she isn't doing anything overt or dramatic. Instead, Jordan commands attention to the point where even a restless leg movement draws the eye and is considered important enough to note.