The completion for sudo with grmls zshrc won't extend to the sbin directories even though Debians sudoers file contains

Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

I suggest to add zstyle ':completion:*:sudo:*' command-path /usr/local/sbin /usr/local/bin \
                 /usr/sbin /usr/bin /sbin /bin /usr/X11R6/bin

to Grmls zshrc (the line is copied from the default Debian configuration)