  The ability to detect liquid argon scintillation light from within a densely
packed high-purity germanium detector array allowed the GERDA experiment to
reach an exceptionally low background rate in the search for neutrinoless
double beta decay of $^{76}$Ge. Proper modeling of the light propagation
throughout the experimental setup, from any origin in the liquid argon volume
to its eventual detection by the novel light read-out system, provides insight
into the rejection capability and is a necessary ingredient to obtain robust
background predictions. In this paper, we present a model of the GERDA liquid
argon veto, as obtained by Monte Carlo simulations and constrained by
calibration data, and highlight its application for background decomposition.
