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.
I feel you have to be careful not to go nuclear, because if he is your professor you have to be careful not to move from "annoyance" to "threat" in his eyes, given he presumably grades your work. However, it seems like he's already put himself in a bad position since you said he "tries ... to humiliate me in front of other professors", not just in class.

I believe to reduce risk of this going nuclear, you have to present as though you're concerned and not accusing.

So go to his boss. Say you're trying to look out for the organisation as a whole. Point out he's breaching privacy laws by repeatedly bringing this up in front of other professors, and if anyone makes an official complaint for any reason then things aren't going to end well for anyone. Ask his boss if they can have a quiet word to "nip this in the bud" and mitigate reputational risk to the organisation, and by implication also to this guy's boss.

Play up your concern for the organisation and try not to get personal. Mention some of the other staff members he's done this in front of. Stress that you believe it's better for all concerned if this just quietly stops happening.

Afterwards, write down what you believe was discussed in the meeting as clearly and succinctly as you can. Send an email to the person you've just met with, thanking them for their time and including your notes on the "informal meeting" (that is, you're explicitly not making a formal complaint right now) "just for their reference" and to "clear things up if I misinterpreted or misunderstood anything".

Assuming this person's boss doesn't argue your email, you've now established yourself on record as trying to deal with it quietly without reputational risk to the organisation. If it escalates or you need to file a formal complaint later, you've established the moral high ground.