You are not allowed a calculator when taking the GMAT. In fact, don't be surprised if your test administrator insists that you leave cell phones, watches, and other devices that could contain a calculator in the car.
You are allowed to use scrap paper and a pencil to figure your math. Take advantage of this. Like other admissions exams, the GMAT is timed (you only have so long to answer all the questions). But unlike other tests, remember that, on the GMAT CAT, once you answer a question, it leaves your computer screen for good. You can't go back to it. You have to answer the questions in the order they are presented to you as opposed to saving the hardest for last.
Some students might argue that these rules make taking the GMAT more nerve-racking than taking other standardized tests. There's a reason for this madness: The business schools you're applying to want to evaluate how quickly you can collect your thoughts and how well you work under the stress of a deadline — both of which are critical in the business world.