JK Rowling never wrote exactly what the Law states, so I can't give you that, but we can assume it from the content we have related to it. If not being able to create food out of nothing is an exception, and the wiki page also assumes that transforming animals to humans is one of the exceptions too, then it seems that the law says that "anything can be transfigured".
It seems to be something like Newton's third law that governs physics "For every action there's an equal and opposite reaction". In the case, Gamp's law would look something like "Any matter can be transfigured into anything" or "Anything can be created", having then its five exceptions of things that can't be transfigured.
Sorry for not giving something solid, but speculation is all we can have in this particular subject.