How would CASPA calculate GPA if you went to a college that doesn't utilize letter grades?

It's my understanding that CASPA relies on letter grades to calculate GPA and doesn't consider 'pass/fail' courses. My issue is that I went to a somewhat alternative college that uses narrative evaluations rather than grades; each class is technically pass/fail and the professor writes a one-two paragraph description of your performance. Has anyone dealt with a similar situation? I am having trouble finding anything online that speaks to this specific dilemma, I'll definitely have to contact CASPA directly but I was wondering if anyone has any other advice? Thank you!