Hi,
because of the way Ethernet works, there is no "gateway" in normal Ethernet. If you need to hide Ethernet MAC addresses, you need to add some other technology.
The easiest solution is to use layer 3 over the link, because this replaces the MAC addresses of the end systems with the MAC addresses of the gateways.
If you really need layer 2, you can look into layer 2 tunneling technologies, but I am not aware of any supported by the Summit X250e switch. Other Extreme Networks switches provide quite a few tunneling mechanisms, e.g. VPWS and VPLS over MPLS, VXLAN, or L2GRE.
Thanks,
Erik