A flexible, free-standing, thermostable gel polymer electrolyte based on plastic crystalline succinonitrile (SN) and ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) entrapped in copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) is prepared and optimized for application in solvent-free solid-state supercapacitors. The synthesized gel polymer electrolyte exhibits a high ionic conductivity over a wide temperature range (from ∼5 × 10−4 S cm−1 at −30 °C up to ∼1.5 × 10−2 S cm−1 at 80 °C) with good electrochemical stability window (−2.9 to 2.5 V). Thermal studies confirm that the SN containing gel polymer electrolyte remains stable in the same gel phase over a wide temperature range from −30 to 90 °C. The electric double layer capacitors (EDLCs) have been fabricated using activated carbon as active materials and new gel polymer electrolytes. Electrochemical performance of the EDLCs is assessed through cyclic voltammetry, galvanostatic charge-discharge cycling and impedance spectroscopy. The EDLC cells with the proper SN-containing gel polymer electrolyte has been found to give high specific capacitance 176 F g−1 at 0.18 A g−1 and 138 F g−1 at 8 A g−1. These solid-state EDLC cells show good cycling stability and the capability to retain ∼80% of the initial capacitance after 10,000 cycles.