Centos 3.6 Help

[tam]

Hi all
I have download Centos3.6 server ISO and installed it, but it seems doesn't have
gcc compiler included in this load. I tried to do "yum install gcc" but it failed. The error msg is like this:
#
Gathering header information file(s)
Server: CentOS-3 - Addons
Server: ....
.....
Finding update packages
Downloading needed headers
Resolving dependencies
...Dependencies resolved
[will do the following:
[deps: libgcc 3.2.3-56.i386
[deps:......
....
[deps:...
Is this ok [y/N]:y
Downloading Packages
Getting libgcc-3.2.3-56.i386.rpm
libgcc-3.2.3..... 100%|=======================| 60 kB 00:00
warning: rpmts_HdrFromFdno: V3 DSA signature: NOKEY, key ID 025e513b
Error: Could not find the GPG Key necessary to validate pkg /var/cache/yumbase/packages/libgcc-3.2.3-56.i386.rpm
Error: You may want to run yum clean or remove the file:
libgcc-3.2.3-56.i386.rpm
Error: You may also check that you have the correct GPG keys installation
[root@localhost root]#

I need the compiler because some software when I install and run ./install.sh,
it will invoke gcc for compiling code.

The site Centos.org has Centos-3.6 with 3 disks labeled CentOS-3.6-i386-diskc1,iso, 2, and 3 respectively. Would these disks when install would have gcc compiler already built on it?
If not, where to get gcc and how to install gcc on the CentOS Server ISO?
Thanks for you guys helping on this matter.
Tam