Here is some more information and maybe this will simplify and answer your question:
Mellanox Gateway product is basically a proxy-arp device (as far as IP goes); means - it will route packets in and out of the InfiniBand network based on source and destination ARP - packet by packet.
it has no awareness at session level therefore, it is not aware of streams. It can only forward back and forth IP packets (of any kind) which means that RDMA will necessarily stay in the IB fabric.
hope this helps..