/usr/lib/gcc/x86_64-linux-gnu/6.3.0 is in gcc-6-base 6.3.0-18+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc/x86_64-linux-gnu/6
/usr/lib/gcc/x86_64-linux-gnu/6