How can I compensate for my low GPA which is 7.5 for me to get a PhD position in physics in the top 30 universities in the USA?

Firstly, you'd want to make a list of the universities where you wish to apply. http://Topuniversities.com can be of some help here. As far as GRE is concerned, most of the physcis grad schools require a near perfect score in the quantitative section. So you'd be looking somewhere around 167-170 on the new scale.  The average verbal score for physics grads on the new scale is around 159. The toefl requirement varies for each grad school. But anything around 105-110 is a good score since it'll strengthen your application for the TA and RA assistantships. The number of internships solely depends on you. But what is important is to do quality work in those internships. Most of the students carry forward the work done in their internships in the form of publication or a paper presentation in international conferences. It is not mandatory to have a publication but most of the applicants do have 1-2 publications along with paper presentations in conferences. Most importantly, you need to have good references. So talk to your professors/supervisors about your future plans and make sure you get a good reference. And it need not be from someone highly accomplished. It  could be from someone who knows you well and can support you whole-heartedly. However, I'd strongly recommend you to improve your cgpa to 8 or above. http://www.physicsgre.com/viewtopic.php?f=3&t=975 This might be of some help. Good luck