I assume that when you say your general gre score is 850, that is the sum of the quantitative verbal sections. Especially since your degree is not in physics, I think you would have to demonstrate knowledge of physics by getting a high score on the physics gre.
I would think that it would be very difficult for even a domestic applicant with a physics degree to get accepted to schools like Penn State with GRE scores like yours. At Penn State, the average Physics GRE score is 73% (~770) and the average quantitative is 86% (~760). And for international applicants, it is usually necessary to get higher scores than domestic aplicants.
Especially since you say you want to study gravitation, which is pretty theoretical, I would say you should try to improve your physics credentials before applying again. Or, you might be able to get accepted to a less prestigious school. There are almost 200 colleges in the US with physics PhD programs.