Statistical Prediction Rules Out-Perform Expert Human Judgments
Unfortunately, GPAs can lie. You cannot be certain of the quality of the problems and evaluation that was averaged to produce the GPA. So running your own test of known difficulty works well to verify what you see on the resume.
For example, I have to hire programmers. We give all incoming programmers a few relatively easy programming problems as part of the interview process because we've found that no matter what the resume says, it's possible that they actually do not know how to program.
Good resume + good interview result is a much stronger indicator than good resume alone.