This hole-in-a-wall is "The Box," and you'll eat well
Spelling Korean food in English is notoriously difficult: gimbab, kimbop, kimbab, kimbap, gimbob--all are different renderings of the seaweed-wrapped rice-veggie-and-meat-rolls that make up Seoul's most convenient quick meal. The streetside stalls are plentiful and cheap, but sometimes it's worth tucking into an alleyway to find a more finely crafted version.
Thus, "The Box," on the ground floor of the building ("SK Hub") that's on top of the Anguk subway station, just east of the Jongno police station. Go down the alley between the two buildings and look for the sign on the left. About halfway between Changdeokgung Palace and Gwanghwamun Plaza, this is one tiny eatery--nowhere to sit, just a counter where you can stand; most get their lunches to go. The kimbap are creative--blanched and seasoned garlic stems! wild mushrooms!--and the design of the packaging is cheerful.
The upper floors of the building where "The Box" is located house a number of studios available for short or longer-term rental. (Think airbnb.) If you end up staying here, "The Box" is an ideal source for a quick-but-well-done lunch to take upstairs to enjoy from your room-with-a-view. Vegetable-wraps and 'ddeok-bok-i' (stir-fried rice-cakes in a sweet-and-spicy red pepper sauce) are also on the menu.