The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca2.5Sr0.5GaMn2O8 has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO6 octahedra approximately 50 K above the 3D Noel temperature of similar to150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO4 layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn3(+) and Mn4+ cations in a 1:1 ratio, and the behavior in the region of the Noel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO4 tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.