A single-crystal diffraction study of the tetragonal to orthorhombic phase transition of GdFeAsO is presented. By cooling below the structural transition temperature splitting of the Bragg reflections was observed corresponding to the formation of four different twin domain orientations. A model was developed to quantify the distortion of the lattice from the position of the split reflections relative to each other. Constrained 2D-Cauchy fits of several split reflections provided the positions of the reflections. The influence of the structural distortion can already be surmised by structural fluctuations in the tetragonal phase before a peak splitting is actually observed.