The mod_auth_gssapi is an Apache module designed to provide Kerberos
authentication to the Apache HTTP Server. This module has been built
as a replacement for the aging mod_auth_kerb. Its aim is to use only
GSSAPI calls and be as much as possible agnostic of the actual
mechanism used.

You will need to add the following line to /etc/httpd/httpd.conf:
  Include /etc/httpd/extra/mod_auth_gssapi.conf