A novel hexagonal boron nitride nanosheets (h-BN)-doped poly (vinyl alcohol)-sulfuric acid (PVA-H₂SO₄) gel polymer electrolyte (GPE) is first fabricated through a facile freeze-thaw method. The ionic conductivities of the h-BN-doped PVA-H₂SO₄ GPEs with various h-BN nanosheet contents are evaluated. With the optimal content of h-BN nanosheets (0.025 mg ml⁻¹), the ionic conductivity of the GPE can reach up to 29 mS cm⁻¹, three times higher than that of GPE without the h-BN nanosheets (9 mS cm⁻¹). The h-BN nanosheets are considered as a “super-highway” for ion transport in PVA-H₂SO₄ GPE. Moreover, a symmetric qusai-solid-state supercapacitor, with activated carbon as electrodes and the h-BN nanosheets-doped PVA-H₂SO₄ gel polymer as an electrolyte and separator, delivers a high specific capacitance, good rate capability, and excellent cycle stability. At the current density of 0.5 A g⁻¹, the symmetry qusai-solid-state supercapacitor can deliver an electrode specific capacitance of 124.5 F g⁻¹, and remain 99.2% capacitance after 5000 charge-discharge cycles. The h-BN nanosheets-doped GPE, with superior performance and facile synthesis method, appears to be a promising candidate for high-performance qusai-solid-state supercapacitors and other electrochemical devices, such as rechargeable batteries, fuel cells. This work also sheds light on the possibility to use 2D materials in advanced GPE.