You cannot quantify it. The chances of outright rejection (as in the case of internships) is much lower but it contributes to your impression in the team. A lower ranker in such firms would be presumed to be getting the opportunity after pulling some strings. However the negativity associated w such a presumption can be easily done away with by bare minimum professionalism: coming in on time for everything, leaving at an acceptable time (after 60-70% of your team and the person you are working with has left or has said that you may leave), avoiding WFH, well structured/formatted submissions (however minor they may be), email etiquette, positive/polite demeanour, etc. You will have to appear compliant.

Lastly, for PPOs, in my personal opinion what is being considered is aspects which a firm will not really be able to teach: soft skills, personality, behaviour etc. If you have basic understanding of the concepts and a brief overview of the practice area, most work is so repetitive that it can easily be taught (which thr firm will do). But if they are not confident of putting a person in front of a client, they wont hire that person.

In my experience, it is unlikely that your CGPA will make a tangible difference in the process of hiring. Depends much more on whether the team likes the quality of your work, work ethic and personality. Unlike campus placements, where firms hire in large numbers without pre-determining which team you'll be assigned to, PPOs are generally offered by specific teams. Resultantly, the focus isn't on ensuring that you're the best candidate on paper - but that you fit well into the team's culture and someone at the Partner/PA level recognises your work.

That being said, your CGPA may be relevant in some other ways. For instance, while I was recommended for a PPO by a Delhi-partner, my PPO interview was conducted by a Mumbai-based partner. I was required to send in my CV in advance, which of course indicated my grades. While I was not asked any questions related to it, it remains difficult to ascertain whether it made any tangible difference in the Mumbai-based partner's approval for my candidacy. In firms (like Trilegal - notorious for offering call-backs/interviews to a large number of students) where the interview is consequential and not merely formal, your chances may improve with better grades.