More details about this building can be found on Wikipedia. The building is now in use by the Baptist University as Chinese medicine clinic, and regular guided tours are given there.
The cache is a 300ml former peanut butter container, well hidden at a point where you can see Lui Seng Chun very well. There will be many muggles at times at GZ but don't worry, they'll only stay until the light turns green (if they care to wait that long).
GPS is very jumpy as usual in urban settings; refer to hint if you have a problem locating it.