I seemed to have accidentally stumbled upon the problem. So I had Taxi2Gate's VHHH installed as well. When I deactivated the scenery (VHHH and VHHH-Terrain) from the P3d scenery list, UGCX was able to recognise the airport and its gates properly.
I only had time for a flight and that worked, but I'm pretty optimistic that UGCX works there.
However, the pushback I performed froze at the end (so the ground crew refused to disconnect unless I killed all operations). I am using PMDG 747 QOTSII and I know it's a problem that's being looked into so that's alright.