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: 
There are several possibilities. Some may be appropriate. Some may work. Or not. 

**Avoidance**. If possible, just avoid this person. Don't have anything to do with him. Difficult, I know. There are probably limited options to do that. 

**Ignore his taunts**. My guess is that he disgraces himself when he does this. If he does this publicly, other students probably see it for what it is. But a public, angry, response from you would probably do yourself more harm than it is worth. 

**Formal Complaint**. This will have consequences all 'round, but might be effective. His department chair might be interested to hear what you have to say, especially if the professor is un-tenured. But a complaint from a group of students would be more effective than one from a single student. And make the complaint in person or using a formal mail. Email is too easy to ignore, for this. 

Try not to feel bad. The actions of the prof are inexcusable and aren't due to anything in you or that you can actually correct. Know it for what it is: unprofessional behavior.
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.