Subway makes almost anywhere possible ... that being said, the walkable older city zones are more in the Chungmuro and Myeong-dong areas in my experience, easy access to Namsan Park, and very short subway rides to many close by market districts, etc. Kinda depends what sort of environment you're looking for. The Gangam district seemed to be predominantly high rise office parks, etc ... no doubt some high end shopping in there if that's of interest. But you're a longer subway ride from most other parts of the city.