To set the default gateway:

route add default gw ip-address interface

For example, route add default gw 10.1.1.1 eth0

To view the current routing table:

route -n
Here is a typical routing table:

Kernel IP routing table
Destination    Gateway    Genmask         Flags Metric Ref  Use Iface
10.241.1.118   0.0.0.0      255.255.255.255 UH    0     0    0  eth1
192.168.1.203  0.0.0.0      255.255.255.255 UH    0     0    0  eth0
127.0.0.0      0.0.0.0      255.0.0.0       U     0     0    0  lo
0.0.0.0        192.168.1.1  0.0.0.0         UG    0     0    0  eth0

The entry with a destination of 0.0.0.0 is the default gateway.