Você veria esse comportamento se seu gunzip tivesse alias para gunzip -N e o nome do arquivo original fosse filename-1 . Para ilustrar:
$ ls
filename-1
$ gzip filename-1
$ mv filename-1.gz filename.gz
$ gunzip -Nv filename.gz
filename.gz: -0.5% -- replaced with filename-1
De man gunzip :
-N --name
When compressing, always save the original file name and time
stamp; this is the default. When decompressing, restore the
original file name and time stamp if present. This option is
useful on systems which have a limit on file name length or when
the time stamp has been lost after a file transfer.