If the system is fighting you trying to use the gift card in a booking with an infant, it looks like you can book online w/o the infant, but then call to add the infant information.

Infant Guidelines

An infant is allowed to travel free of charge on the lap of an adult from seven days old until his or her second birthday. Even if your infant is flying for free, there may be taxes due when traveling internationally. International taxes will be collected at check-in.
Book all travelers EXCEPT the lap infant online then contact Reservations at 800-432-1359 to add the lap infant to the reservation.