Students' self-efficacy in physics classes can play a key role in shaping course outcomes. Prior research has shown that women have a lower self-efficacy than men in calculus-based introductory physics courses. We administered a validated survey to 564 students to investigate the gender differences in self-efficacy and how it predicts student grades at the end of a mandatory two-semester introductory physics course sequence primarily for bioscience majors in which women outnumber men. We used structural equation modeling to investigate how students' self-efficacy predicts female and male students' grade at the end of the course. We found that women had a lower self-efficacy and grade than men and that the students' self-efficacy played a major role in predicting students' grade even though women outnumbered men in this physics course. This study hints at the fact that numerical representation alone is not sufficient, e.g., to mitigate the effects of deep-rooted societal stereotypes and biases related to who belongs in physics and can excel in it. Thus, it is important for physics instructors to be intentional about creating equitable and inclusive learning environments in which all students, particularly those from traditionally marginalized groups such as women, have high self-efficacy and can thrive.