It sounds like you are trying to upgrade an old 2-wire thermostat to a digital one. Unfortunately, it is not always straightforward to determine which connections to use when wiring a new thermostat. It is important to double check that the wiring is correct before connecting the new thermostat. It is highly recommended that you consult a qualified HVAC technician to ensure that the wiring is done correctly. Additionally, they will be able to advise you on the best type of thermostat to use for your gas furnace.