Hi
try to install the yum-metadata-parser rpm you can find it here
ftp://ftp.muug.mb.ca/mirror/centos/5.5/os/x86_64/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpmyou can install it using
rpm -Uvh yum-metadata-parser-1.1.2-
3.el5.cent
os.x86_64.
rpm
and then run
yum clean all
yum groupinstall "gnome-desktop"
try to remove any unnecessary repos , just keep centos base repos
hope it works :)