/usr/lib/x86_64-linux-gnu/libgmp.so is in libgmp-dev 2:6.1.2+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgmp.so.10.3.2
/usr/lib/x86_64-linux-gnu/libgmp.so.10.3.2