Seoul has two 'real' Chinatowns:
1) Garibong-dong (가리봉동), down the hill from near Guro (구로) stn exit 3. This is the larger of the two, and insular to the point that not many Koreans, much less other tourists, venture here. It's also under threat from a giant redevelopment project that's been imminent for several years, but I was unable to find any confirmation that it's actually started.
Note that these are both pretty low-key collections of shops that cater to Chinese immigrants, and if you can't read the signs, they doesn't really look much different from any semi-suburban neighborhood in Seoul — but if the restaurant has its signboard in Chinese characters, it'll be authentic alright. Possibly too much so, Daerim in particular has a fair number of restaurants that serve/specialize in dog (狗).