Example of a GGSL simulation. Left panel: HST image of the cluster M1206 at z = 0.439 (∼2′ across), with the critical lines (in white) at z = 2.5 from the lens model (Bergamini et al. 2019). A specific secondary critical line is marked in red, with a zoomed-in image shown in the upper left ∼10″ inset. The green spot indicates the position of the corresponding caustic on the source plane. Central panel: source plane at z = 2.5 showing the caustic (in red) corresponding to the selected critical line, including the buffer (black dotted line) delimiting the injecting region; the injected source has a Sérsic profile (index n = 1.5, R_eff = 0.14″), mag_F814W = 26.3, and the SED of a star-forming galaxy. Right panel: colour-composite image of the resulting simulated GGSL system, together with the critical line (red dotted line, with a circularised θ_E = 1.7″); the cutout is ∼10″ across.