It depends on the current skill set of the individual. For few it’s easy, for others it’s not. The syllabus is limited to basic level Math and English but the questions are tricky and one cannot afford to get too many questions wrong if the target score is high.
One good thing about GMAT is that its pattern is more or less fixed. Same set of concepts are checked and the scope of syllabus is limited. So, if you put efforts in the right direction and refer to the right material, you can easily do well at it.
Caution: Please do not be distracted by plethora of free/paid material available on the net. You can score well by doing only 1% of it and you may jeopardize your chances at GMAT by solving even 50% of it. What’s important is not the number of books/study material you finish…what’s important is referring to the right material at right stage in your preparation and learning from every page that you read. Experts’ Study material will make you do just that!