Before you log-in GDAC, you need to check lists at below.
1. Do you have your own [Kakao Talk application] as below mobile?
GDAC is able to sign up with just a Kakao account, but for log-in you need Kakao talk application for receiving a certification number
2. I don't use [Kakao Talk application].
You might not able to use Kakao Talk, in this situation the 'log-in certification number' will be sent to your mobile as (MMS&SMS).
(If it's not shown at your message(text)list, please check the spam folder)
※extra case : if you have just deleted your Kakao Talk, log-in certification number may not shown normally
after trying all these solutions and you are still having problems with it,
please contact our support website link below.