The development of high-performance gas-gap heat switches is crucial for improving the refrigeration performance of mK sorption coolers. The effects of structural dimensions as well as cold- and hot-end temperatures on the heat-transfer performance of gas-gap heat switches were calculated and analyzed in this study, and relevant experimental studies were conducted. The results show that the heat-transfer performance of the gas-gap heat switch in the ON state primarily depends on the thermal conductivity of the gas in the gap. Reducing the gap size can improve the temperature gradient of the gas in the gap, and the thermal resistance of the heat switch operating in the 4-10 K range is reduced to 15 K/W. Additionally, the theoretical switching ratio can reach 1100 when the gap size is reduced to 0.1 mm. The thermal insulation performance is primarily affected by the thermal conductivity and wall thickness of the support tube, with the wall thickness exerting the most significant effect. Additionally, increasing the operating temperature can enhance the thermal conductivity of the heat switch but deteriorates its thermal-insulation performance. Based on these findings, a gas-gap heat switch with a switching ratio of 822 is developed.