I have a very similar setup but with a wireless access point connected to the gateway, which means I can have just one network. Your solution may be as simple as enabling ip_forward on your debian laptop (ie; echo "1" > /proc/sys/net/ipv4/ip_forward). If this works and you want it to persist across reboots you can set this up in /etc/network/options. Let me know how you go. Darryl