Gwangju is easiest to navigate when you treat the subway and city buses as one connected system rather than separate tickets. Gwangju Transportation Corporation says the metro uses a flat fare, and its current fare page lists the adult transportation-card fare at KRW 1,250 and a single-use ticket at KRW 1,400.
The important part for visitors is transfer behavior. Gwangju’s official fare guidance says subway, city bus and shuttle-bus transfers are free when made within 30 minutes after getting off, but the benefit requires a transportation card. Cash and single-use metro tickets do not qualify for the transfer discount. You also need to tap the card when getting off to preserve the transfer benefit.
The same official guidance distinguishes wider transfers involving rural buses from Naju, Damyang, Hwasun, Hampyeong and Jangseong; those can have a different discount structure. If your route crosses the city boundary, do not assume the ordinary Gwangju-only rule applies.
For a simple visitor routine: keep enough balance on one transportation card, tap in, tap out, and recheck the exact bus or train before leaving a major attraction. If several cards are kept together in a wallet, the operator warns that payment may fail or be charged incorrectly.
Practical next step: use the official Gwangju Transportation Corporation fare and timetable pages on the travel day, especially for your last train or any route that crosses into a neighboring county.