To my knowledge, there are no significant benefits on AA from holding the no-fee Citi credit card, besides 25% off AA in-flight food and beverage purchases charged to the card.
UA actually does not provide expanded award space to the United Gateway Card, according to
a TPG article about expanded award space for credit card holders.
I assume you have a Citi card because Barclays does not offer a no annual fee AA product. Citi has some pretty lenient product change rules so you don't have to stay in the AA card family -- have you considered product changing to a DoubleCash for instance?