Our forked version of Gzip adds a patch to add the --rsyncable option, which creates an "rsync-friendly" archive. This option used by vzdump utility
Here is the full description of this option:
"While compressing, synchronize the output occasionally based on the input. This reduces compression by about 1 percent most cases, but means that the rsync program can take advantage of similarities in the uncompressed input when syncronizing two files compressed with this flag. gunzip cannot tell the difference between a compressed file created with this option, and one created without it."