gzip now accepts the --rsyncable option. This option is accepted in
all modes, but has effect only when compressing: it makes the resulting
output more amenable to efficient use of rsync. For example, when a
large input file gets a small change, a gzip --rsyncable image of
that file will remain largely unchanged, too. Without --rsyncable,
even a tiny change in the input could result in a totally different
gzip-compressed output file.

http://savannah.gnu.org/forum/forum.php?forum_id=8495

Please use --rsyncable whenever you use gzip (provided it's recent enough), namely when creating yum repo metadata files.