Due to the low out-of-band emission and robustness against asynchronous transmissions, generalized frequency division multiplexing (GFDM) has potential to be a new modulation scheme for the foreseeable applications. In this paper, a novel precoding scheme with tunable band sizes is proposed for GFDM systems. The precoder and equalizer are jointly designed aiming at minimizing the system total mean squared error (MSE). Simulation results show that the proposed precoding scheme with small band sizes can provide symbol error rate (SER) close to that with full sizes.