T-money is an all-around transportation card used for public buses, subways, taxis, and convenience stores that accept T-money as a form of transaction (most do). The function of a T-money card is like a debit card, where money is deposited into the card and used to get around the city of Seoul and the Gyeonggi-do district. Can I use my T-money card for the all stop train to the airport?
Yes, according to the Korean Tourism Organisation
Transportation card purchase locations: Transportation Center on B1F of Incheon Int’l Airport Terminal 1 & Terminal 2 (Single-journey transit card), convenience stores inside Incheon International Airport (Tmoney card), Tourist Information Center near Gates 5 & 10 on 1F of Incheon International Airport Terminal 1 (MPASS)
That's advice for arriving passengers. They also list it here:
Transportation card information: Tmoney / Exclusive Tourist Cards