This is intentional.  gpart is:

    ExclusiveArch: %{ix86}

In the spec file.  x86_64 users are expected to use the i386 build, which will work just fine because x86_64 is really a multiarch platform.  On RHEL-5, x86_64 users really should also have an i386 repo enabled because of the number of things that are built i386 only but are intended for x86_64 as well.

But by default there is no i386 repo configured by an install of an x86_64 server and the x86_64 repo contains i[36]86 rpms as well. It should not be necessary to pollute a standard RH yum config by configuring non-standard repos surely?

[root@xxx ~]# yum repolist
Loaded plugins: changelog, dellsysid, downloadonly, protectbase, rhnplugin, security
repo id                             repo name                                                         status
dell-omsa-indep                     Dell OMSA repository - Hardware independent                       enabled:    596
dell-omsa-specific                  Dell OMSA repository - Hardware specific                          enabled:      6
rhel-x86_64-server-5                Red Hat Enterprise Linux (v. 5 for 64-bit x86_64)                 enabled: 10,125
repolist: 10,906
[root@xxx ~]# yum list gpart
Loaded plugins: changelog, dellsysid, downloadonly, protectbase, rhnplugin, security
Finished
0 packages excluded due to repository protections
Error: No matching Packages to list
[root@xxx ~]# yum list firefox
Loaded plugins: changelog, dellsysid, downloadonly, protectbase, rhnplugin, security
Finished
0 packages excluded due to repository protections
Available Packages
firefox.i386                                     3.6.11-2.el5                                    rhel-x86_64-server-5
firefox.x86_64                                   3.6.11-2.el5                                    rhel-x86_64-server-5
[root@xxx ~]#

(In reply to comment #1)
> This is intentional.  gpart is:
> 
>     ExclusiveArch: %{ix86}
> 
> In the spec file.  x86_64 users are expected to use the i386 build, which will
> work just fine because x86_64 is really a multiarch platform.  On RHEL-5,
> x86_64 users really should also have an i386 repo enabled because of the number
> of things that are built i386 only but are intended for x86_64 as well.

Huh? A distinctly dubious comment.

Please remember, this is RHEL 5 and not Fedora.

I'm well aware that this is RHEL.  As stated, x86_64 users are expected to use the i386 build of the gpart package.  It will work fine, you already have the i386 libraries on your system given the multiarch nature of the x86_64 platform.  gpart is not the only package like this, there are a number of i386-only packages that are usable on x86_64 systems.

That said, this issue has been brought up before for previous RHEL-5 update releases and I have proposed it as an update, but gpart never makes the approved component list for that release, thus the problem persists.  I do not see this changing unless someone escalates the problem through the technical support channels.  In the interim, feel free to use the i386 gpart package.

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
The gpart package was previously only available for the i386 architecture. This update makes the gpart package available for both i386 and 64-bit x86 architectures.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2011-1376.html