What Type of Test Taker Are You?
Both the GMAT and GRE use computer-adaptive testing systems, but with an important difference: within each section of the exam, the GRE's system allows test takers to skip questions and return to them later; with the GMAT's computer adaptive system, you may not revisit a question once you submit your answer for it.
Bottom line: If you typically perform better on exams where you answer the easiest questions first, and then return to the more difficult ones, the GRE may be easier than the GMAT for you.
On Which Practice Exam Did You Achieve Better Results?
If you have not already taken practice GMAT and GRE exams, do so. This will allow you to learn about the specific content of the exams and see how well you perform on each.
Bottom line: While taking a practice exam without any preparation won't afford you a glimpse at what your actual score would be, the experience may provide perhaps the best indication of which exam truly will be the easier one for you.
Proper Preparation = An Easier Exam
Whichever exam you determine to be the least difficult for you, you can make your selected exam even easier by properly preparing for it.
Utilize an Online Prep Course
A prep course allows you to fully prepare for all sections of the exam you plan to take without having to utilize and compile various study materials. Study.com's online GMAT Prep: Help and Review and GRE Test: Practice and Study Guide prep courses are self-paced, so you may study when and where you have time, and are taught via short, engaging video lessons. Both courses feature self-assessment quizzes and exams to help you gauge your progress and understanding of the content you'll find on your test.
Take Advantage of Tools Offered by Exam Administrators
Administrators of both exams offer free prep software, which is available for download on the official websites for the GMAT (mba.com) and the GRE (ets.org). Depending on which exam you plan to take, you'll also find resources such as sample questions, prep tips, handbooks, and suggested study timelines on these sites.