A mechanically robust gel polymer electrolyte (GPE) with cross-linked networks is crucial for energy storage and conversion applications. Poly[1-cyanomethyl-3-vinylimidazolium bis(trifluoromethane)sulfonimide] (PCMVImTFSI) was treated via various bases at room temperature, leading to GPE membranes with high mechanical strength. The structures of the GPE membranes were characterized by Fourier-transform infrared spectroscopy, solid-state NMR and atomic force microscopy. The “cation–methylene–nitrile” group sequence embedded in the repeating units of PCMVImTFSI enables in situ cross-linking of nitriles under basic conditions such as with mild NH₃ vapor, significantly improving the membrane formation ability of PCMVImTFSI. The maximum fracture strength of the NH₃-treated membrane is 6.2 MPa, 8 times higher than those of membranes that were thermally cured at 200 °C for an equivalent time of 6 h. The crosslinking chemistry is viable with various cations and counter anions of PILs, thus representing a facile and effective method to prepare high-strength GPE membranes.