Correlation between the distance of the closest low-ionization gas clouds and the GRB luminosity. Left panel: Monte Carlo simulation of the L_iso-distance relation. In red the L_iso (erg s⁻¹) and distance (parsecs) values with their error bars for the GRBs with available measurements, while the contours represent the result of the simulation of N = 10000 realizations. Right panel: Probability density function of the Spearman coefficient and the associated p-value for each realization. The black dashed line represents the median value while the dotted ones represent the 2 σ intervals.