gr310437:
They will take your post-grad diploma into account as another data point, but it will not carry as much weight as your undergraduate GPA. The reason being, not everyone who applies with more than a bachelor's degree. So, your undergrad GPA will be weighed against the undergrad GPA of your fellow applicants.
Think of a post-grad diploma along the lines of a CFA/CPA designation or master's degree. It can be an added benefit but isn't a replacement for a core requirement.
Coupled with a good GMAT/GRE score, you can argue in your optional essay that your academic credentials are strong despite a below-average GPA. It's not a panacea but it's better than nothing if you feel you need to give a rationale.
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