I think you are right. The one in Grand Central is the Physical Ladies' Club. I just saw the sign for the gym you are looking for in Shatin and it seems to point to the KCR building up where the old postoffice used to be and is now resturants.
Looked on Yellow Pages
http://www.yp.com.hk and there is no listing except for the Physical Ladies Club and Maru Beauty & Fitness Centre in Shatin Centre Shopping Arcade which I don't think is the one.
May be it has no opened yet. You can bet that when it does there will be hundreds of sales staff all over Shatin trying to sell you membership
