My entire UG transcript was P/F. AMCAS didn't calculate a GPA for me because they couldn't.
Just enter every class you took into AMCAS *exactly* how it appears on your transcript. You can get an unofficial copy of the transcript from each school to do this. AMCAS is going to verify your entries against the transcript, so it will save you a lot of suffering and hassle if you just follow along and copy your transcript class by class.