Hi Yuxi,
Thanks for the reply! This is kind of embarrassing, but after I returned the gates, I felt like I was "cheating", so I re-leased all the gates that I was using - from that point forward, I could no longer return gates that were in use.
BUT
I realized just a few minutes ago that I had missed my gates at Hong Kong - I have been running PEK-HKG x23/wk without renting any gates at HKG. Just now, when I tried to open a route from HKG-NNG, I was warned that I didn't have enough slots at the airports to do so. Once I leased a gate at Hong Kong, I was able to open the flight. Now, I have 1 gate rented at HKG, but it only counts 3 slots in use, which are the flights from HKG-NNG. None of the 23 flights from PEK-HKG are using a slot.