Gongdeok is a big interchange — Lines 5 and 6, the Gyeongui–Jungang line and the AREX airport line all meet here — which makes it a handy place to drop a bag, especially on an airport day.
Coin lockers at Gongdeok Station
Gongdeok Station has T-Locker coin lockers at 2 spots, about 43 in total, on B1 near Exits 5, 6 and 7 (the Line 5 and Line 6 sides). See each spot and how many are free now, by size, on the Gongdeok Station page.
Prices are the standard Seoul rate — about ₩2,200 (small) to ₩4,400 (large) for four hours, then an hourly charge. English screen, card, no ID.
Why Gongdeok is handy for the airport
The AREX runs from Gongdeok straight to Incheon and Gimpo airports. On a last day the clean move is to store your bag in the morning, spend the day light, then collect it and step onto the airport train — no detour to Seoul Station. For a case that won't fit a locker, see sizes and prices.
When they are full
Check live availability on the station page first; if it is full, Hongdae (Hongik Univ.) is also on the AREX and has a much larger bank. The full guide covers your options.
Sources: prices, sizes and hours from the Seoul Metropolitan Government (T-Locker); spots, counts and availability from Seoul Open Data.