/usr/lib/x86_64-linux-gnu/libmrpt-gui.so is in libmrpt-dev 1:1.2.2-1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmrpt-gui.so.1.2
/usr/lib/x86_64-linux-gnu/libmrpt-gui.so.1.2