On gateway start "tcpdump -i xl1" then on client start "ping
192.168.0.1". Do you see the packets?
After you stop the ping run "arp -a" on both client and gateway.
Do you see an entry for the other host?
If the answer to thease questions are "No" then you have a layer
two (or below) problem. Check cabling, vlan configuration and
such.
It the answer to thease questions are "Yes" then you have a layer
three (or above) problem. Check your packet filters.