Motion management for tumor physiological motion has been an important issue in external beam radiation therapy. By integrating a PET system into radiation beam delivery system, RefleXion's biology guided radiation therapy (BgRT) system uses real-time PET emissions from the tumor itself as a biological fiducial to deliver radiation that tracks the tumor. In order to understand BgRT's performance limit and provide guidance for its development of next generation systems and architectures, we built an accurate BgRT PET system model in GATE. A series of validation experiments per NEMA NU-2 2018 standard such as, sensitivity, noise equivalent count (NEC) testing, and image quality analysis, have been conducted in both the real and simulated systems. Preliminary results demonstrated a close match between these two systems.