sed is one of the programs we assume is always present when we build
packages in Fedora which is probably also what is wanted here.  (A
default install of Fedora will include sed but someone might be able to
create a minimal install that did not include it.) Note that within
Fedora we usually use a wildcard with man pages.  For example::

  %{_mandir}/man1/foo.1*

I'd suggest doing this rather than hardcoding ".gz".  Automatic
compression of manpages could be disabled on other distros, set to
bzip2, compress, or xz instead.  Wildcarding the suffix will catch all
of these cases and be more future-proof.