What is the GMAT?
The GMAT is the standardized test for Business school. Like the GRE, it consists of a Verbal and Quantitative section. Unlike the GRE, the two sections are combined to give a composite score of 800.
The GMAT cannot be used in place of the GRE, so unless you are going to business school, do not take the GMAT.
What does business school have to do with the GRE?
Unlike the GMAT, which only those applying to business school sit for, the GRE is accepted by many accredited business schools (ever hear of Stanford or Harvard?). That is not to say all the top business programs accept a GRE score. Always check with the business program to see which ones accept GRE scores.


Is the GMAT more difficult than the GRE?
It depends. That is, it depends on who is taking the test. For those with strong Quant skills the GRE math will seem easy. The GMAT math is definitely more daunting. So if you struggle with Quant the GMAT will seem very difficult.
On the other hand if you struggle with the nuances of vocabulary and style, the GRE verbal section may be very difficult. That is not to say that the GMAT verbal is easier. But the Sentence Corrections section is more of the science of grammar, and those with logical minds tend to be more adept at quickly sifting through a morass of words to find grammatical errors. The GRE Text Completions and Sentence Equivalence questions, by contrast, are testing the art of word usage, a skill most humanities majors have been honing for years.


So which should I take?
For those applying to business school, first make sure that your program accepts the GRE. If so, then ask yourself which test better shows off your strengths. If you are unsure, you can always take an official GRE practice test and an official GMAT test (both are free). If you score significantly better on one, then the answer is clear. If there isn’t much difference, spend a week getting a feel for each test to see which one suits your skill set better.