Under high-voltage operation, the solvents and anions in traditional gel polymer electrolytes (GPEs) are prone to irreversible oxidative decomposition, leading to the formation of heterogeneous high-impedance cathode electrolyte interphase (CEI) film and the dissolution of transition metals. Meanwhile, the liquid components in the electrolyte react violently with the lithium metal to form a fragile solid electrolyte interphase (SEI) film. This exacerbates the uneven ion flux, dendrite growth, and accumulation of "dead lithium", significantly increasing the risk of short-circuiting and accelerating capacity fade. Therefore, a composite gel electrolyte was developed through molecular design and interfacial synergistic regulation strategies, using in-situ copolymerization of polyethylene glycol diacrylate (PEGDA) and 2,2,2-trifluoroethyl acrylate (TFEA) as the framework, and succinonitrile (SN) as an additive. The results show that the -CF₃ groups in TFEA are preferentially reduced to form a LiF rich SEI layer, which has high ionic selectivity to homogenize the lithium-ion flux, inhibit dendrite formation, and achieve stable cycling of lithium symmetric cells for over 1 300 h. On the cathode side, the high-voltage oxidative decomposition of SN generates highly ionic Li₃N, which synergistically constructs a stable CEI layer with the LiF component formed by the oxidative decomposition of TFEA, jointly suppressing the decomposition of solvents/lithium salts and the dissolution of transition metals. The electrochemical stability window of the electrolyte is significantly extended to 4.8 V, and the assembled Li||LiNi0.8Co0.1Mn0.1O₂ cell achieves a high capacity retention of 75.23% (at 0.5 C) after 120 cycles at 4.5 V.