What determines the communication speed? is it the physical link's speed or the communication card speed?
RS232 is an "asynchronous" serial protocol, which means that the communications clock signal isn't directly wired from one unit to the other.  Instead, each unit is "told" the speed at which it should communicate.  If both units are set to 9600 bps, for example, then the receiving unit will sample the data lines at just the right time to intercept the signals coming from the transmitting unit.  This allows bidirectional communication with only two wires (plus ground).

So, for your system, you want to choose any speed that both units can achieve.  The gateway is rated to 115kbps, but you can configure it to operate more slowly.  If the relay only communicates at 9600, then you'll have to set the gateway to 9600, also.