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.
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.