GRE (grduate records examination) is a pre-graduate school exam taken by some who plan on going to grad school. It's very general and not specific to any course of study in undergrad. Check out Wiki:
http://en.wikipedia.org/wiki/Graduate_Record_Examination
As for PT vs med school, which is harder to get in? I would guess med school as the requirements are tougher in my mind. They may take more people but the GRE is nothing compared to the MCATs. Also, many get masters degrees in areas such as public health, etc to boost their chances of getting into a top 10 school (definitely not a typical route for a PT applicant).
Without question...med school is much tougher.
Typically thousands of applicants for 60-200 spots depending on the school. PT schools in the NE I have experience with...sometimes not enough applicants to fill spots....year to year. Expensive education that is little more than a bachelors degree, and relatively low pay.
If you love it - you're lucky.
If your considering PT school....also consider MD/DO/PA/DPM school.
Many more options...especially PA school. Good money and autonomy....and you have the latitude to work in any specialty....that can be nice.
Unedited, ATC/PT1