Kent, from the Taipei Air Station website, just returned from a trip to Taipei. He very kindly took a few photographs of the area around the old HSA compound, including the old north entrance to the United States Taiwan Defense Command. You can click on any of these photos for a larger image.
This first picture is one that I received from old friend Les a while back, which shows the old north entrance to the USTDC compound in the 1970s. You can see part of the building in the background.
This photo is one that Kent sent from his recent trip, and I'm all but positive that it's the same area. Today it's a taxi rest area and you can see that the old building is gone. Just before this entrance and to the right is the entrance to the art museum parking lot.
Here's an overhead image of that area as it looks today and I've circled the general location of the gate and the USTDC building.
The big white building just to the left of the circle is today's art museum and the art park is the grassy area with all the paths to the south.
Here is a map of the art park, identifying the various sections. Please note that it is oriented to the east.