#24732 was closed two years ago as invalid.
The zlib port was updated to 1.2.6 some time ago. It's not surprising they've removed or moved the 1.2.5 distfile. Doesn't matter anyway; MacPorts properly handles 404 errors and retries the download from another server, such as one of the MacPorts distfiles mirrors, which do have that old file.
The MacPorts zlib portfile is not configured to attempt to download from gzip.org.
If you are having trouble downloading files from someone's server, you should report the problem directly to the administrators of that server.