“For the Verbal Reasoning and Quantitative Reasoning measures of the computer-delivered GRE revised General Test, the reported scores are based on the number of correct responses to all the questions included in the operational sections of the measure.”
The extract above is from the website of ETS, the organisation which develops and administers the GRE. Clearly, your score on the GRE depends on the number of correct responses. In addition, nothing is subtracted from the GRE score if you answer incorrectly. This means that you should attempt a high number of questions. In fact, the ETS suggests that it is best to answer every question across both the sections.
Obviously then, you can guess on the GRE. If you have prepared decently enough for the GRE, you should be able to eliminate a couple of answer choices quite easily. Once you eliminate a couple of answers, you can then pick the most probable answer. Since the GRE awards you for correct answers, a few correct guesses will increase your chances of a high score.
However, you should not waste too much time in guesswork. Try to go through the sections rapidly and attempt questions which you are confident about. You can then tackle the more difficult questions or take calculated guesses. The GRE also permits you to skip a few questions and revisit them if time permits.
Remember that the ultimate aim is to do well in both verbal and quantitative areas of the GRE. Since the exam is section adaptive, a poor performance in the first sections will lead to easier questions in the second sections of both the subject areas. Correct answers to difficult questions count for much more than correct answers to easier questions.