The amount of CO generated by gas stoves during the cooking process is negligible, otherwise most homes equipped with natural gas cook stoves would require some form of ventilation near the source, which they don't. This is not to say you can leave all burners running full tilt 24/7, but people have been using gas to cook with in homes for a long time. Opening the kitchen window alone would be more than sufficient to allow exhaust gases to vent while cooking. A fan would be overkill. A fuel leak would be a bigger concern, and as noted by others, is easy to avoid/mitigate.