Which 20 gallon tank is it? 12" tall or 18"?? It only makes a difference because 40 watts of light at 18" is waaaaaya less intense than 40 watts at 12" (minus the gravel depth!).
Either way, you could probably do without co2. It will just mean slower growth and higher possibility of algae. Make sure you really stock up on plants to start, try to get as many as possible.