I dont know what must have popped up this new development, But i think there is a new agreement between Gtbank and Airtel NG. The latest development, affects all Airtel users as they need to have a minimum amount of N5 on their Airtel line before they can carry out USSD transaction with 737
I could remember vividly on UBA, that you are charged as you use the USSD platform. So if you intend buying airtime to your device, you must have airtime on your sim. But according to Gtbank, you are not charged for buying airtime.
This was the message they sent; "Dear Customer, Airtel NG now requires that you have a minimum balance of N5 for USSD transactions. Airtime topup is exempted"

Source:http://www.techfashy.com/2018/01/airtel-users-using-gtbank-ussd-plaform.html