Should I prepare for the GRE if my gpa isn't yet high enough for Epidemiology graduate school?
I went through graduate school in Epidemiology at a major school of Public Health, and am now the epidemiologist on a number of multidisciplinary admission committees.

Improve Your GPA First

There's no subject-specific test for the GRE for Epidemiology (which isn't surprising, as it's not really a discipline with good undergraduate representation), and the general feeling in the field is the GRE is a supplemental piece of information - a way to maybe push an edge case into "Admit", or to provide context for schools where it's known that the GPA doesn't mean much. My program has gone so far as to seriously consider phasing it out entirely.

GPA cutoffs are also often a function of the Graduate School, or another administrative group where there isn't much room for nuance. If you don't meet the GPA requirements, you're sitting squarely in the "Do Not Pass Go" category. I wouldn't bother investing any time or money studying for the GRE until you're past that line.