I am by no means an expert on this, but my take is this -
1. If you are invited, they decided that the GMAT was not bad enough to eclipse the rest of your profile.
2. In the interview, if they ask (or maybe even otherwise), you'll have a chance to explain why you got a low GMAT.
3. GMAT / GPA are somewhat complementary. If you are top of the class at IIT, they know you can handle the course work.
4. At this stage, worrying is not going to get you anywhere. You have an interview. Make the best of it. Focus on your extracurricular activities. Show them that that you can lead. Be gracious.
Don't worry. You stand a good shot. Wish you the best!