In SPECT Monte Carlo simulation the tracking process OF photons inside the collimator is a very tedious and computationally demanding task. For example in the case of a low energy parallel-hole collimator only one in a thousand photons penetrating the collimator surface will be detected. In an attempt to significantly decrease the time associated with the collimator tracking process we implemented within GATE the methodology designed by X. Song et al which relies on the use of Angular Response Function (ARF) for the collimator/detector system. The Angular Response Function refers to the spatial distribution of photons detected for a given point source position. The strategy developed by Song relies on the use of different Monte Carlo codes such as MCNP and SIMSET in order on one hand to generate list mode files from which the ARFs are computed and on the other hand to compute the ARF tables from these list modes files.