If your system is connected to the Internet using either
PPP, then don't set the default route in
Linuxconf. Just select the default route option of the
specific connectivity package you are using (dip, pppd, slirp).
Those packages will setup the default route when the link is up
and remove it when it goes down. With
PPP, IP numbers are
often allocated dynamically from the other side, so you have little
idea of the gateway IP until the link is up.
On complex networks with many LANs, with many gateways and even redundant gateways, your best bet is to use dynamic routing with packages like routed (supported by Linuxconf) or gated.