If you enter a default gateway in linuxconf, and you do "activate configuration", the new setting doesn't work. after doing a "route" i see that the route to the default gateway isn't in there. I usually reboot (the windows NT method ;-), and it works then...but I guess it is possible to add it manually in the route table. Actually it's rather confusing that almost every change works immediately, except this one...
The default route is only set when the interface that the route is on is brought up. Take the interface down and bring it back up (something which should get done if you change the interface's address or netmask), and your route should get set.
Every time I use Linuxconf to edit any parameter, my routing table is EMPTIED, with the exception of the localhost. I then lose all network and internet connectivity, and it is impossible to add a route via the route daemon. The only solution I have found is to use netconfig. HOWEVER, watch out because the default gateway and nameserver are auto-filled incorrectly!
I'm going to close this bug since we don't ship linuxconf anymore (thank heavens).