I took the GRE because it was required for OSU's perfusion MS program which I applied to a few years ago. I didn't take any subject tests since it wasn't required and I didn't want to waste the $, but the general GRE was a joke. You can't really study for it since it's so general, but maybe get a $50 Kaplan GRE book just to familiarize yourself with the format and the buzzwords for the Verbal section synonyms/antonyms or basic math formulas (shape area/length forumula, triangle angle rules, etc) if you're rusty.
During the test, I felt like I was taking the ACT or SAT again. The math is nothing but general algebra and geometry. The test is computerized, and questions are supposed to get harder or easier based on how you are performing. The questions were all so basic (triangle geometry, "solve for x," etc); I thought I must be doing horrible because they weren't getting any harder lol. It turned out that I scored perfect on the quant section, but 5% of all science and engineering students do (many top engineering programs expect their applicants get an 800 in quant). The Logic is pretty easy also... I think I got 90th+ percentile (720). Verbal threw me a bit of a curveball, and I only scored mediocre (580... a bit above where average science majors land and roughly where the average English major would sit). Then again, I only scored about average on MCAT Verbal (8), so maybe it was an indicator.
From talking friends from undergrad who have taken the exam more recently for engineering, nursing, neurobiology, etc grad school apps, I think the GRE has changed since I took it and scrapped the Logic section in favor of a writing evaluation. However, the bottom line is that the test is nothing compared to the MCAT. Even if the science subject-specific tests are harder, I still wouldn't expect too much of a challenge.
Have you already taken the GRE or are you signing up for the GRE because your "backup" grad schools require it? If you haven't taken it yet, I'd consider saving your money and just taking the MCAT; any science grad program (Bio or Physio MS, SMP, PhD, etc) should know how to evaluate MCAT scores and may be willing to accept you based on that exam alone.
If you are looking for a prognostic indicator of how you will do on the MCAT, you will have a pretty darn good idea once you do enough practice tests. The old ones from the AAMC are great (expensive, but well worth the $), and Kaplan practice tests from the book or course are solid also. If you are consistently scoring, say, 24-26 on the recent AAMC practice tests during the month before your testing, I'd expect very minor, if any, variation on the real exam.