Question: I am a beginning graduate student in Mathematics. I have a professor who always brings up the fact that my GPA is not great in my undergraduate degree even though my performance is above the norm in the institution I am enrolled at the moment, so far I got straight A's in the graduate courses that I have completed. He always tries to point out at my flaws in my undergraduate years to humiliate me in front of other professors and my colleagues. What would be a wise reply/reaction to such abuse? 

I feel bad about it cause I feel like I am harassed and I don't reply.
I am also currently taking a course with that professor.

Answer: 
Seek advice from a trusted mentor.

If you have another professor or advisor who you believe you can speak to about this, I would encourage you to explore that option. Not only is it important to have a positive influence to counteract the negative impact of this professor in your personal development, but quality mentorship is also a component in your future career success. 

A mentor who has already passed these trials and tribulations in their career may have very well witnessed and experienced these same behaviors. They can offer a more informed plan for how to treat this with your best interests in mind. What your mentor advises may boil down to the same options @Buffy has laid-out. In the case that you should choose formal action, a mentor supporting you in this could be very influential in how it concludes. 

One of the unfortunate realities of academia currently is that institutional mechanisms to discourage and rectify this type of behavior are (in my opinion) rare and frequently ineffective. For better or worse, your professors often have an inordinate amount of influence on your career once you are at the graduate level. For this reason, it's really difficult for a student to utilize formal recourse options. Some schools do have specific [anti-bullying resources](https://graduateschool.vt.edu/student-life/we-hear-your-voice/disrupting_academic_bullying.html), and you should investigate if these exist at your institution.
It would appear that he is trying to shame you to make himself feel better. This is not unusual behaviour, though you would hope for better from a senior academic. One solution is to call him on it enough to stop it but not enough to make an enemy of him: 

"Yes, my GPA was indeed 3.0. That seems to have really caught your attention because you've mentioned it the last 9 times we've met."

I would not try to emphasis that you are getting great grades now. The important bit is to call him on his GPA-focus, particularly if there are other people around. Do it in a manner which suggests that you are genuinely curious about why he is mentioning it, rather than letting him know that it irks you.