show gateway IP address when performing ifconfig command

You can with the ip command, and given that ifconfig is in the process of being deprecated by most distributions it's now the preferred tool. An example:

$ ip route show
212.13.197.0/28 dev eth0  proto kernel  scope link  src 212.13.197.13
default via 212.13.197.1 dev eth0

Run:

$ route

The output is:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         10.0.2.2        0.0.0.0         UG    1024   0        0 eth0
10.0.2.0        *               255.255.255.0   U     0      0        0 eth0

No, there isn't. According to the man page you can't modify the output of ifconfig (except showing disabled interfaces, too).