X-ray topography was used for a comprehensive study of elastic strain and dislocation-induced relaxation of GaAs layers grown on (100) Ge substrates. Measurements of tetragonal distortion and radius of curvature as well as direct transmission imaging indicate that the GaAs layers are highly metastable. Observations also indicate the existence of two sources of misfit dislocations, the threading dislocations and some stacking faults. Threading dislocations are activated first. Their activation involves plastic deformation of both the epilayer and the substrate. Stacking faults are activated later. This unusual misfit dislocation nucleation mechanism is discussed.