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PET image quality can be significantly affected by respiratory motion artifacts. 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. As a result of the non-uniform geometric sensitivity, the count rate from a given organ, will depend on the axial location of the organ. Consequently the respiratory phase can be determined from count rate changes which are determined by suitable temporal resolution from the list-mode data stream. The GSG method has several advantages over existing methods; it only uses LOR events. It is non-invasive, no additional hardware device systems and patient preparation required. Using GATE (GEANT4 Application Tomographic Emission) and NCAT (NURBs(Non Uniform Rational B-Splines) Cardiac Torso) software packages, realistic simulations of respiratory motion demonstrate that GSG can be used for respiratory gating. The validation on clinical data demonstrates that GSG is able to reduce respiratory motion artifacts.