This option started as an out of tree patch which at least Debian carried for a long time before it got merged upstream in GNU gzip 1.7. For a while the dpkg perl modules (used for the package building machinery) used the --rsyncable only conditionally whenever the system was Debian based. But I removed that given that GNU gzip 1.7 has been around for some time now. And then I got a report from Alpine Linux which seems to be using busybox, and was now failing. It would be nice to have this option implemented which helps with generating artifacts that can be more easily rsync'ed (and thus is potentially easier on mirrors), I'd also appreciate not having to look for a way to support the different implementations while not having to run something like «gzip --help» at run-time whenever the perl module is first loaded which might incur a too heavy cost.
Do you have a patch?