/usr/include/singular/gfanlib/ is in libsingular4-dev-common 1:4.0.3-p3+ds-5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/singular/gfanlib/_config.h
/usr/include/singular/gfanlib/config.h
/usr/include/singular/gfanlib/gfanlib.h
/usr/include/singular/gfanlib/gfanlib_circuittableint.h
/usr/include/singular/gfanlib/gfanlib_matrix.h
/usr/include/singular/gfanlib/gfanlib_mixedvolume.h
/usr/include/singular/gfanlib/gfanlib_paralleltraverser.h
/usr/include/singular/gfanlib/gfanlib_polyhedralfan.h
/usr/include/singular/gfanlib/gfanlib_polymakefile.h
/usr/include/singular/gfanlib/gfanlib_q.h
/usr/include/singular/gfanlib/gfanlib_symmetriccomplex.h
/usr/include/singular/gfanlib/gfanlib_symmetry.h
/usr/include/singular/gfanlib/gfanlib_traversal.h
/usr/include/singular/gfanlib/gfanlib_tropicalhomotopy.h
/usr/include/singular/gfanlib/gfanlib_vector.h
/usr/include/singular/gfanlib/gfanlib_z.h
/usr/include/singular/gfanlib/gfanlib_zcone.h
/usr/include/singular/gfanlib/gfanlib_zfan.h