/usr/lib/x86_64-linux-gnu/libgtk-3.so.0 is in libgtk-3-0 3.4.2-7+deb7u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgtk-3.so.0.400.2
/usr/lib/x86_64-linux-gnu/libgtk-3.so.0.400.2