The best advice ever got for these kind of things is, walk into the interview with the confidence that you already have the job/position. I do not mean walk in there arrogant and condescending, rather with the confidence that not only do you deserve to be there but you deserve the position you're interviewing for. If you do this you will find yourself not only less nervous, but more engaging (not to mention come across wayyyy better to the interviewer).
As for the GPA, you're right it will most likely come up, but you have the advantage of knowing this walking into the room. Just be up front about it, it is one thing to try to spin your reasons for the GPA in a more positive light, it is another to lie, be aware of this. You got an interview because someone in admissions saw something in your application that they like, you would not be granted an interview if they thought you couldn’t pass their curriculum. So I would work on an explanation for your GPA (without making it too much of a sob-story) and just be up front about it, part of what they are seeing is how you would react to these kind of things. Most likely they will bring it up, just be honest about it and try to put it into a positive light and move the conversation to a topic your more comfortable talking about. Best case scenario is they don't bring it up.
Just be confident, and be prepared. Good luck.