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Three-dimensional topological insulators (TI) have emerged as novel states in condensed matter physics. The surface states of a 3D TI are composed of odd number of massless Dirac cones in a Brillouin zone. The existence of these Dirac cones on the surface is characterized by the Z₂ topological invariant. Strongly spin-orbit coupled material Sb₂Te₃ and most of its doped materials belong to the Z₂ topological-insulator class. Using a combination of first-principles calculations, magneto transport properties and angle resolved photoemission spectroscopy (ARPES), we directly show that Gd_xSb_2-xTe₃ is a large spin-orbit-induced indirect bulk bandgap magnetic material whose surface is characterized by a single topological spin-Dirac cone. We demonstrate that the dynamics of spin Dirac fermions of Sb₂Te₃ can be controlled through Gd doping, making this magnetic material classes potentially suitable for topological device applications.