Hello Cliffness,
Both GRE and GMAT, as most standardized tests do, test the fundamentals of Math and English. There are certain distinct differences in the test pattern. It is difficult to ascertain if your GMAT prep would suffice for the GRE without taking a practice test. GRE Quant section is easier than GMAT Quants. The greatest difference is that GRE has a built in calculator on the Quant section. Furthermore there are Data Interpretation questions along with the traditional topical questions in the Quant section.
You can easily evaluate your current level by taking one of the tests freely available online. Here is a list of available tests.
free-gre-practice-tests-506.html?fl=menuBest of luck with test preparation.
Regards,