hey, it seems we have a similar background (you can look at my profile on the Fall 2019 application profile thread to get an idea of the schools that I applied to and the ones I've gotten in). I also got a BS in math and physics and want to do mathematical physics research. My GPA is a bit over 3.5, but my mGRE score was not great. Still, I was able to get into two fairly decent schools so far and am still waiting for responses on some other good schools. While your GPA is indeed a handicap and might even keep you from qualifying to some schools that set lower bounds on GPA for admission, it's still definitely possible to get into a good school if you get a really good GRE score and have really good letters of recommendation. If you aren't able to obtain these then I would probably look for funded masters programs. If you know who your recommenders will be, a good starting point would be to ask them where they have collaborators or connections and look into applying to those places. Sometimes a letter from a prof connected to the department is all you need. One thing I did to even out my chances was to look at the AMS department groupings and applying to an even amount of group I schools and group II schools while at the same time making sure there would be people at these places that I would be interested in working with. Mathematical physics is a broad and insanely active field so chances are many of the places you look into will have at least 2 or 3 people interested in it. Of course, if you know what flavor of mathematical physics you want to do then your choices will be narrowed down.