/usr/bin/mkgrkindex is in texlive-lang-greek 2017.20180305-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../share/texlive/texmf-dist/scripts/mkgrkindex/mkgrkindex
/usr/bin/../share/texlive/texmf-dist/scripts/mkgrkindex/mkgrkindex