A Zemax/Matlab Monte Carlo (MC) simulator of a gamma detector module for PET applications is presented. The simulator models the optical arrangement and the photodetector based on CMOS Single-Photon Avalanche Diodes (SPADs). The simulator serves as a tool for optimization of the detector configuration, as it allows the designers to evaluate different sensor configurations and find the one that maximizes the signal-to-noise ratio (SNR). Three sensor configurations have been tested, employing SPADs with a diameter of 8 µm, 16 µm and 32 µm. Results show that 16 um-wide devices lead to the best trade-off between fill factor and noise.