Note that gateways and exchanges aren't the same thing, though a gateway could easily choose to be an exchange and vice versa. Ripple isn't really a network to connect either exchanges or gateways. It serves as a distributed exchange all by itself, and the gateways serve to connect the other nodes in the network, the network doesn't serve to connect the gateways.
Of course a gateway is the same as an exchange... What on earth would you use a gateway for other than to exchange your money for another currency? OK, sure, you just want to send dollars to someone, so you go to "gateway"... the gateway exchanges the dollars to some electronic currency... the recipient collects in dollars at some gateway close to them.... the money is being "exchanged". Every gateway is definitely an "exchange".
It should be noted, that presently, in order to send money accross the globe to your friend, you simply go to an exchange, change your money into bitcoin, and then send it to your friend who exchanges the money back to his currency of choice. This is done completely using exchanges. In Ripple, you could argue that this money is sent by IOU. But still... come one... this IOU thing is still basically what currency is in the first place... ultimately, in ripple you have two new currencies introduced, ripples, and IOUs...