The gateway address is not needed to "ping" from a computer located on the same network with the two boards.
If you want to communicate outside your network then the gateway address should be the address of a router or internet gateway. The gateway address should not be the same as either embedded board.
After you ping each board you can tell how the Ethernet MAC address was set using the "arp" command.
Even if you get no response to "ping" there still may be an ARP entry for the board. If you see an ARP entry then the board is communicating on Ethernet.
The Ethernet MAC (hardware) address is usually set from an EEPROM connected to an Ethernet chip. In the case of your embedded boards the Ethernet MAC address is probably compiled into the Ethernet driver. You should be able to set the MAC address using a kernel parameter for the Ethernet driver. Look at the documentation for the Ethernet driver or look at the source for the Ethernet driver to find the driver parameters.
Are you able to get one board to work by itself? Does one board respond to "ping"? Before you try to make both boards communicate, verify if you can communicate to each board individually. Also, check the MAC address for each board using "arp".