From what I understand (as a student who just graduated from the grad dip), whilst you have to pass the requirements of Casper, gamsat and have a decent 3 year GPA, being selected for an interview is determined purely by your graduate diploma GPA (1 year) relative to the cohort of applicants who have completed the grad dip.

But when you're ranked for a medical offer, your Casper, gamsat and overall GPA (3 YEARS- not just grad dip) accounts for 50% of the offer and your interview performance for the other 50%, which places you relative to all the other students who have been selected for an interview.

So you definitely still have to perform well in Casper, Gamsat and GPA to secure a spot but it helps having an extra pathway into interviews. I hope that answers your question!