Respiratory motion artifacts can be a significant factor that limits the PET image quality. To improve image quality, surveillance systems have been developed to track the movements of the subject during scanning. Gating techniques utilizing the tracking information, are able to compensate for subject motion, thereby improving lesion detection. In this paper, we present a gating method that utilizes the geometric sensitivity gating (GSG) of a 3D-PET scanner system operating in list event acquisition mode. GSG method has several advantages over the existing methods, it only uses LOR events and is non-invasive, no additional hardware device is required and there is no additional patient preparation required. Using GATE (GEANT4 Application Tomographic Emission) and NCAT (NURBs(Non Uniform Rational B-Splines) Cardiac Torso) software packages with an Allegro PET configuration, realistic simulations of respiratory motion demonstrate that GSG can be used for respiratory gating.