Will your odds be better with a higher GPA? Yes. Will a perfect physics GRE score make up for it? Maybe, maybe not. Depends on the school you apply to. In most cases, I would say, "No." The physics GRE is important, and it's in your best interest to do well, but it's not that
So, based on my own applications and what the trends seem to be from my classmates and other people on this board, the typical admissions weighting probably looks something like this:
- Recommendation letters are the most important.
- Research is nearly as important. Unless you're applying for a very competitive field or to a very elite institution (and sometimes not even then), it's generally more important to have a lot of quality research than it is to have worked in the field you're applying for. Publications are a big plus, although not strictly necessary.
- Your statement of purpose comes next. The SOP accomplishes a few things: it's an opportunity to demonstrate your interest in physics, brag about your accomplishments, and describe why you're interested in a particular school and/or researcher. This last point is particularly important, as getting accepted or rejected from a top school can often boil down to whether or not someone is interested in working with you.
- After that, your grades are important. Specifically, your grades from your upper-division physics coursework. A lot of schools really don't care what your grades were like in a freshman history class, but they most certainly do care what your grades looked like in classical mechanics.
- Now the physics GRE is important. It's important enough that bad score can really hurt your chances for a good school, but it's not important enough that a high score guarantees you a spot in Harvard.
So, let's talk a few more details about your GPA. Do your grades trend upward or downward, i.e., are your lower grades mostly from classes you took as a freshman and the like, or are they more recent? If it's the latter, are they in your upper-division physics coursework? Would any of your recommendation letter writers be likely to address the low points in your grades and vouch that you actually know the material? If you answered, "Downward, yes, and no," then I would say your shots at an top school are pretty low, although not impossible. If you answered, "Upward, no, and yes," then good research credentials (with accompanying recommendations), a good PGRE score (900+), and a strong statement of purpose will probably be enough to give you decent odds at a top 20-30 school and about as decent odds as anyone else at an elite school (so, not good, but still possible if the admissions committee is in a forgiving mood that day).