We compute temperature-dependent electronic mobilities in GaN through Monte Carlo simulations. Bulk material having ionized impurities, and a quantum well structure are examined. Our values show good agreement with recently obtained experimental data on bulk GaN only when space-charge scattering is taken into account. Though a simple form for the internal potential variation has been used, more realistic space-charge distributions could easily be incorporated into the present simulation scheme. Mobilities for GaN quantum wells have also been obtained by explicitly taking account of interface roughness scattering. The predicted values yield a theoretical upperbound fairly close to measured data.