Symbolism, Imagery, Allegory
Gemma doesn't just have the normal emotional turmoil that most teenagers face. No, Gemma has emotions that are showing up for the first time ever, so they're embarrassingly strong and difficult to handle. She even bursts into tears at school:
I really struggled to keep all of my newfound emotions under control. There was this one awful incident at school when I had this sudden outburst and started bawling right in the middle of Mr. Belford's lecture on Plate Tectonics. People stared at me like I was a freak, which is understandable. I mean, only a freak would cry over shifting plates. (2.11)
Gemma is embarrassed and concerned by her emotions, but they symbolize the human part of her. This is the part of her that Stephan, Sophia, and Marco have tried to tamp down (because they're afraid it will burn out the star's power), but when Gemma's feelings resurface anyway, much as it may overwhelm her, it's also a sign of the strength of her humanity. All these magical folks haven't actually been able to make the human parts of Gemma disappear.
As we meet her, Gemma is finally getting to live life as a human being complete with all of its ups and downs. Having emotions isn't a piece of cake—and sometimes it's pretty freaking hard—but it's a million times better than just being dead inside. That's why Gemma is so upset when Stephan says that he's going to make her emotionless again: She doesn't want to go back to a robotic and uninterested existence.
To explore Gemma's feelings further, swing by her page in the "Characters" section.