You are not going to end up with a GPA for graduate school unless your graduate school calculates one for you. Look at your transcripts and see if this is the case. If not, send a letter to your registrar and see if they will do a grade conversion for you. In many cases: Honors = A; High Pass = B and Pass = C but your school may award more points for Honors and High Pass.
In any event, pose this question to the AMCAS folks and let them tell you how they handle your graduate grades. You can do this by e-mail.
The good thing is that graduate school counts about as much as an extracurricular activity. The bad thing is that if your graduate work is not distinguished and your undergrad is below average, graduate school becomes more of a liability than an asset.
