No, cutting off the gas supply to a stove will not reduce cooking time. In fact, it will have the opposite effect. When the gas supply is cut off, the stove will no longer be able to provide heat to the cooking surface, which means the cooking process will come to a halt.
Cooking time depends on the heat applied to the food. By cutting off the gas supply, you are removing the source of heat needed for cooking. If you want to reduce cooking time, you would typically increase the heat output of the stove to cook the food faster. Different stoves have various heat settings, and using a higher setting or increasing the flame will generally speed up the cooking process.
Keep in mind that adjusting cooking times and temperatures can vary depending on the recipe and the type of food being cooked. Always follow the recommended cooking instructions in recipes to ensure the best results.