Staying calm is, in my opinion, one of the most important things in the GMAT actual test. If you fall short of time, you risk to switch to "panic mode", which in most of the cases means that your brain will struggle to understand even the most basic questions.
Know yourself and know how much time do you need to stay calm during the whole exam.
For example, If you know that you usually need some extra time in Quant section, guess 1 or 2 questions around questions 10-20 (the first) and/or 20-30 (the second). You can perfectly assure a Q50 doing so.
If you guess a question, the probability that it will hurt your score is "only" 60%, considering that there are 3/4 chances the question is not experimental, and 4/5 you pick a wrong answer (3/4 * 4/5 = 6/10 = 60%). In addition, if you spend some seconds to make an
educated guess, then that probability will decrease.