Certain systems, such as amphiphile solutions or diblock copolymer melts, may assemble into structures called ``mesophases'', with properties intermediate between those of a solid and a liquid. González-Segredo and Coveney were able to simulate the assembly of a gyroid cubic phase from a blend of two immiscible fluids and a symmetric amphiphile, using a multicomponent Lattice Boltzmann algorithm. Recently, as part of the TeraGyroid project, we performed extremely large-scale simulations of the assembly of this mesophase, in order to observe the dynamics with reduced finite-size effects. For sufficiently large system sizes, it was possible to observe defects and grain boundaries in the liquid crystal structure. We describe the various sorts of defect observed, and the behaviour of these defects as the gyroid phase assembles.