Given that we already use `gzip` in dired for other formats like `.tar.gz`
and `.tgz`, could we not just replace gunzip with `gzip -d`?  I mean,
even in my (gnu/linux) system `gunzip` is just a script that passes
through to `gzip -d`.

I don't think that change would be a breaking one because we already
assume that `gzip` is present in the system and that `gzip -d`
decompresses files (again, the command for `.tar.gz` is "gzip -dc %i |
tar -xf -").  But if it is considered breaking, we could maybe add an
option to only use gzip?