You haven't told us when you're gonna graduate. Anyways to answer your queries:
1) GPAs are very important for graduate school admission. Even though your major is in a different field, a low GPA is definitely not going to look good on your profile. Try and get it above 8.5+ if possible. Making excuses that you were interested in learning physics is not a valid excuse for messing up your undergrad GPA. Trust me that number is going to stick to you for a long time whether you like it or not.
2) Regarding research experience, try looking for summer projects at IISc/TIFR or other research institutions in India. They'll give you a taste of real physics research and also get you some strong recommendations if you do well there. But to build up a strong profile you should preferably spend every summer possible at some research institution or the other. There are some official internship programs too but they do have a GPA cutoff.
3) PGRE: Get it out of the way as soon as possible, hopefully by third year itself. The concepts are pretty simple and can easily be mastered in a couple of months at most. Trust me, this would save you a lot of time in your final year.