The development of safe lithium metal batteries (LMBs) is critical for practical applications with high-energy-density demanding. In this study, a phosphorus-containing diethyl vinylphosphonate (DEVP)-based gel polymer electrolyte (PD-VI GPE) with high ionic conductivity of 6.38 mS cm⁻¹ is prepared by in-situ γ-ray radiation polymerization. The PD-VI GPE induces the formation of a uniform, dense fluorine- and phosphorus-rich solid electrolyte interphase (SEI) in Li||Cu coin cells, effectively suppressing interfacial side reactions and enabling stable lithium deposition. Pouch cells assembled with the PD-VI GPE (2 g Ah⁻¹) exhibit a specific energy of 420 Wh kg⁻¹ with 89% capacity retention over 80 cycles. A novel in-situ separator thermal shrinkage assay reveals that the PD-VI GPE-coated Celgard separator maintains structural integrity at 129 ℃. Phosphorus-functional groups in the PD-VI GPE act as oxygen radical scavengers, inhibiting cathode-derived O2 evolution in abusive conditions. Thus, LMBs assembled with the PD-VI GPE demonstrate suppressed thermal runaway and mechanical abuse tolerance. This study establishes a material design paradigm that concurrently addresses interfacial stability and safety challenges, paving the way for the application of LMBs in energy systems with high safety requirements.