completely agree with mNeo. In some ways both of them test the same skills, but they are very different. I took the GRE about 7 years ago, and a lot has changed since then. But I would say, you will need to
1) Both GRE and GMAT verbal have Reading Comprehension and some analytical questions. Other than that GRE tests vocabulary while GMAT tests grammar.
2) GRE problem solving is much easier than GMAT. And GRE doesnt have Data Sufficiency, which can get tricky. So if you have already given the GMAT or are preparing from GMAT books, GRE quant may look easier. Still, you should not restrict your preparation to
Manhattan GMAT books..
3) Both have essay writing now, and I believe they are not very different (somebody correct me if I am wrong on this)