So, the problem here is whether the employer’s decision is a case of religious discrimination. Technically, I do not think this is discrimination because she was not singled out because of her religion. It would be discrimination if other people were let off but not her. As a matter of fact, this is more of an example of religious equality: No matter what you believe in, no break means no break. That being said, the question comes down to whether she has the religious PRIVILEGE to be allowed to take those days off. According to Bader & Wilkinson (2019), Religious Freedom Restoration Act provides protections for individuals to exercise their religion. This also includes their ability to seek reasonable accommodations from their employers. So, she might have the privilege because of her religion to be treated differently and be allowed to have the break which ultimately is a form of religious discrimination not against her, but against all non-Roman Catholic workers.
In short, I think this is not a religious discrimination case, but a religious free act case. It may seem that the ask she has is reasonable, but before I proceed, let me bring a few hypothetical examples forward. Say in one case, one person is in a religion that requires sacrifices. I am certain the law would not allow the killing of another person just because of religion. Well, that is a harmful exercise and very extreme. Let’s take a step back. According to Green (2019), Muslims have a specific praying time of 04:29 PM. Assume a devout Muslims surgent decided to pray at that specific time that ultimately led to the death of a patient. Praying is not a harmful exercise but the outcome turned out to be dreadful. I am sure this is also not acceptable. I think you are probably seeing where I am getting at. So, it is not the act, but the consequence of that act that really determines if the situation is reasonable to have that time off. Back to Gina. Prior to her leaving, there is no telling what the potential consequences are. Furthermore, she does not have to go to that place because The Catholic Church had not designated the site an official pilgrimage site. She decides to go there nonetheless despite there being other options. Technically, this is no different than someone leaving work unauthorized just to see his/her favorite band and then getting fired.
Knowing the company policy, she could have communicated with the manager sooner and tried to work out an alternative plan. Seeing that there is no effort to effectively communicate from her end, I do not think she has a case against her employer. However, since she did feel the ‘calling’, she can always try to file a lawsuit against Virgin Mary for getting her fired.

References:
Bader, C. D., & Wilkinson, W. J. (2019). Religious freedom, LGBT rights, and the prospects for common ground. Cambridge University Press.
Green, M. A. (2019). Employment discrimination law: Cases and materials (3rd ed.). Wolters Kluwer