  We report on exceptionally long spin transport and giant spin lifetime
anisotropy in the gapped surface states of three-dimensional (3D) magnetic
topological insulators (MTIs). We examine the properties of these states using
the Fu-Kane-Mele Hamiltonian in presence of a magnetic exchange field. The
corresponding spin textures of surface states, which are well reproduced by an
effective two-band model, hint at a considerable enhancement of the lifetime of
out-of-plane spins compared to in-plane spins. This is confirmed by large-scale
spin transport simulations for 3D MTIs with disorder. The energy dependence of
the spin lifetime anisotropy arises directly from the nontrivial spin texture
of the surface states, and is correlated with the onset of the quantum
anomalous Hall phase. Our findings suggest novel spin filtering capabilities of
the gapped surface MTI states, which could be explored by Hanle spin precession
measurements.
