The Kanohi Garai would control weight and gravity, not mass. It would reduce the amount of molecules you have in you, allowing you to become less dense. It could allow you to become intangible, so that you could float through walls, it could allow you to float on water by making yourself less dense than water, you could turn yourself into a meteor of sorts and become so dense that it would be impossible to move you. There are many other things you could do, but these are some of them.