  We investigate the magnetic excitations of elemental gadolinium (Gd) using
inelastic neutron scattering, showing that Gd is a Dirac magnon material with
nodal lines at $K$ and nodal planes at half integer $\ell$. We find an
anisotropic intensity winding around the $K$-point Dirac magnon cone, which is
interpreted to indicate Berry phase physics. Using linear spin wave theory
calculations, we show the nodal lines have non-trivial Berry phases, and
topological surface modes. We also discuss the origin of the nodal plane in
terms of a screw-axis symmetry, and introduce a topological invariant
characterizing its presence and effect on the scattering intensity. Together,
these results indicate a highly nontrivial topology, which is generic to
hexagonal close packed ferromagnets. We discuss potential implications for
other such systems.
