We only managed to visit at night. The street food location is at gate 8. Some of the stalls have long long queue. The watermellon drink stall seems to be a hit... A bit pricey at KRW9000 for one watermellon drinks (2 cups equivalent).
Other food is spaghetti, korean fried chicken etc. Grab the food you want and there are sitting area to eat.
There are also few restaurants selling raw fish and sashimi at the same place