There is an agregator bestrate.org which provides you with the best rate. Indeed, I've checked the rate of GNT into ETH. The main advantage of it you don't have to compare the rate among different exchange projects. Just in case, if somebody decided to make exchanges
GNT is on all major exchanges and I don't feel there is a need for an exchange on any such new sites which may not have the reputation and are easily created for scams.