Heat pumps are way more efficient at producing heat than gas heaters. Ideally with a large temperature difference between the starting pool temperature and the pool temperature that you want, you would run both at the same time. Although the heat pump would be adding less heat, it would be doing so at a lower cost. As you approach the pool temperature that you are trying to achieve, the gas heater would drop out. If the heat pump could not maintain the desired temperature, the gas heater would kick back in. I have worked in the controls side of HVAC for many years and many dual fuel home heating systems are controlled in this manner. Controls wise you can even set a variable temperature band between the point where only the heat pump is running and both are running depending on how your controls are set up. Initially dual fuel home systems were set up to disable the heat pump below 30 degrees, but current thinking has lowered the cutoff to 0 degrees or less. Although the heat pump output is much less at lower outdoor temperatures, it is still much more efficient than gas heat.