Lattice Monte Carlo (MC) simulations in the NVT ensemble together with a coarse-grained model of the block copolymer chains are used to explore the phase diagram of pure and bidisperse diblock copolymer melts as a function of temperature and block volume fraction. In the pure systems, we found that the gyroid phase is stable in only a narrow region of the phase diagram. Through the analysis of the structure of the channels and nodes formed by the minority component in the gyroid phase, we found evidence of "packing frustration" of the chains inside each such node, manifested as a central low-density region. The use of chain-length bidispersity was then investigated as a way to reduce such packing frustration in the gyroid phase. We found that the longer chains in such systems tend to segregate preferentially inside the gyroid nodes. For a system with components with a 2:1 ratio of chain lengths, we observed an increased range of temperatures where the gyroid phase is stable.