/usr/lib/x86_64-linux-gnu/libgsmext.so is in libgsmme-dev 1.10+20120414.gita5e5ae9a-0.3+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgsmext.so.1.0.4
/usr/lib/x86_64-linux-gnu/libgsmext.so.1.0.4