How do I change IP and gateway addresses in Linux. I'm currently using Redhat

Which distro are you using?

In redhat I think the script for that is in /etc/sysconfig/network-scripts. For eth0 edit the ifcfg-eth0 file. Is it on dynamic assignment at the moment?

Run the following command in a terminal window:
ifconfig
Look at the line that has "Link encap:Ethernet". The alphanumeric right before that -- in my case "p5p1" -- is the suffix of the file you want to edit in /etc/sysconfig/network-scripts. In my case that's /etc/sysconfig/network-scripts/ifcfg-p5p1. Open a text editor like "gedit" and open the aforementioned file. Edit the line that says "GATEWAY=" with an ip address at the end by changing it to whatever gateway ip you want to use for the gateway. You might also need to edit /etc/sysconfig/network the same way. Restart networking.
EDIT: I forgot to mention changing your ip is done with the same files. Don't forget you have to either restart networking or reboot for the changes to take effect.

you can use the setup command in command line .

it will give you the option to change the ip address and DNS as well..

In redhat you can also use "system-config-network"

It ubuntu you need to edit /etc/network/interfaces file and below lines

# vi /etc/network/interfaces
auto eth0
iface eth0 inet static
address x.x.x.x
netmask x.x.x.x
network x.x.x.x
broadcast x.x.x.x
gateway x.x.x.x