Magnetic properties of Ga_1-xMn_xN are studied theoretically by employing a tight binding approach to determine exchange integrals J_ij characterizing the coupling between Mn spin pairs located at distances R_ij up to the 16th cation coordination sphere in zinc-blende GaN. It is shown that for a set of experimentally determined input parameters there are no itinerant carriers and the coupling between localized Mn^3+ spins in GaN proceeds via superexchange that is ferromagnetic for all explored

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... _ij values. Extensive Monte Carlo simulations serve to evaluate the magnitudes of Curie temperature T_C by the cumulant crossing method. The theoretical values of T_C(x) are in quantitative agreement with the experimental data that are available for Ga_1-xMn_xN with randomly distributed Mn^3+ ions with the concentrations 0.01 ≤ x ≤ 0.1.