The sentence that shows that Gabriela has self-confidence is the following:
"I'd be a good Juliet, too, and I'd be the sun, just like Romeo says, glowing, shining—his light in the dark".
In this sentence, it is clearly stated that Gabriela has self-confidence because she is expressing her inner belief in herself. She firmly expresses that she "would be a good Juliet" and that she "would be the sun...glowing, shining". She is full of confidence about being good for the part of Juliet.