Aqueous zinc metal batteries (AZMBs) suffer from dendrite growth and parasitic side reactions, limiting their lifespan and stability. While in situ gel polymer electrolytes (GPEs) form robust electrode-electrolyte interfaces, conventional approaches typically require chemical initiators and crosslinkers that impose harsh conditions, generate byproducts, and result in discontinuous Zn²⁺ conduction pathways. Here, we report that simple mixing of sulfobetaine methacrylate with an aqueous ZnSO₄ solution spontaneously induces self-polymerization, enabling an initiator- and crosslinker-free in situ self-polymerizing GPE under room-temperature and ambient-air conditions without external energy. Zn²⁺ lowers electron density at the vinyl group and induces monomeric aggregates, triggering self-polymerization and rapid formation of a physically crosslinked GPE. The resulting network provides intimate electrode-electrolyte contact and continuous Zn²⁺ conduction channels, achieving a high Zn²⁺ transference number (0.76). Zn||Zn symmetric cells employing this GPE suppress dendrite formation and exhibit stable cycling for over 4100 h and reliable low-temperature (-10°C) operation, significantly outperforming conventional initiator- and crosslinker-based in situ GPEs. Zn||VO₂ full cells also exhibit excellent cycling stability with ∼96% capacity retention. This ZnSO₄-induced in situ self-polymerizing GPE strategy embodies green chemistry principles and enables a high-performance, cost-effective, and sustainable solution to zinc anode challenges, paving the way for next-generation AZMBs.