If you get stuck on a GMAT question or simply don't know the answer, guess. The computer-adaptive GMAT requires that you answer every question.
Many people who take standardized exams don't get the score they could achieve because they spend too much time dwelling on a single question, leaving insufficient time to answer other questions they could get right. Don't let this happen to you. Never spend more than a minute and a half to two minutes on any one question. With enough practice, you'll almost automatically know when a problem is taking too long. If this happens, try to eliminate some choices, and then take a guess. The key is not to get stuck!