Well although no gum is allowed according to GMAT policy, if you can discreetly and quietly do it, you will be fine. They will not kick you out of the test for having it, they will only make you spit it out. If you feel that it gives you an advantage, I would consider using it. But make sure to use it during study and practice tests and practice on being
completely silent. They don't allow gum b/c they don't want you to bother other test takers, or have anything that could help you cheat along with you. So as long as you are discreet, it shouldn't be a problem. And worst case scenario, they will just make you spit it out.