I'm going to be a senior college student in the fall. My gpa is close but not high enough for the bare minimum graduate school admission criteria (gpa=3.0). I am wondering whether I should wait until I finish senior year and see whether I get a cumulative gpa of 3.0 before I study for GRE/apply for graduate school or whether I should study for GRE now. The disadvantage of studying for GRE without the GPA criteria fulfilled is that I might end up studying for a useless test.
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.
I would not spend time and money in my final year of college studying for the GRE if there was anything more than a small chance I'm not applying that cycle.
Put that energy into improving your GPA and getting relevant research experience. The GRE and planning materials aren't cheap (it's a racket anyway), and it will be easier on your wallet if you wait until next summer, when you have a full-time job.
Besides, instead of studying for three classes and the GRE, you can focus only on the GRE after you graduate.
And in any case, GRE scores are valid for five years:
For tests taken on or after July 1, 2016, scores are reportable for five years following your test date. For example, scores for a test taken on July 3, 2018, are reportable through July 2, 2023.
I doubt that studying for the GRE would be useless if you do so effectively. Among other things, it should help you consolidate what you have learned generally.
However, as you suspect, studying for your courses should come first for a lot of reasons, not just your GPA. But don't waste any odd moments during the year doing unproductive things. When you have a few minutes (waiting for a bus, say) have something to study.
Also, take a lot of notes (paper is preferable) which helps you remember and operationalize what you know.
It is also pretty common for a student to get distracted in the final year. Don't let that become a problem.
Focus on repairing GPA first. Here are a few things that could be helpful when your admission profile reaches the committee: i) your last year's GPA shows an uptrend, ii) you have taken some courses related to epidemiology or public health and did very well, and iii) if there is a serious dip in your GPA some where along the study in the transcript, you have provided a fair clarification in the statement.
So, if credits allow, consider cross-register into some intro-level public health course to see whether the topics speak to you, and work hard to get good grades.
If for some reason you created a gap year/semester while studying for GRE, consider using that chance to work with some researchers, even as data collectors or data entry staffs, to get some field experience. This is particularly helpful if your resume does not have any noticeable relevance to health care, public health, or epidemiology. In additions, be a professional worker and cultivate a good relationship with the team; they could potentially write you a powerful and relevant reference letter.
Also, if you are in the US, I am pretty sure some students with GPA lower than 3.0 did get into public health graduate programs. With public health programs popping up here and there, there may even be more chances. In the US, public health programs are mostly accredited so in terms of contents they don't vary a lot. So, look carefully what the faculty work on and see if their topics speak to you. Sometimes the works that attract you may not be in the top-tier institutes.
Lastly, broaden the options. Do a brief job search on epidemiology-related works, you'll see that this field hires diverse kinds of specialists. For example: economist, policy scholars, psychologists (esp. quantitative psy., questionnaire development, etc.), nurses, computer programmers (data informatics and analytics), statisticians, biochemists, toxicologists, etc. Hopping into one of those options, and then get a Master in Public Health part-time while working may also be a way.
Accepting the answer of Azor Ahai, I want to add that studying for GRE is not something lasts for months. Those exams of ETS and any other similar tests have always some key tips, in which you learn them and in a short while you get satisfactory results. I, for instance, used GRE for my applications 3 times, I made quantitative part full, but no one made a fuss over it. It just takes sufficient amount of focus. For verbal part, reading English novels, espcially classic ones help a lot to get the vocabulary. For writing part, I can't say anything but there will be people around you who can at least direct you to improve your writing while improving your GPA at the same time.
GRE assesses everyday skills, so if you manage your own business, here improving GPA, in an efficient way, there will be no need to study GRE, still easy to say as its cost is quite high, though.