I was just wondering when it says next to my gate when I arrive - Gate will be available shortly
Using standard AI traffic, either default or 3rd party, that message usually means the ETD for the AI that has taken that parking is very short, like 1 minute or so. When the waiting time is longer, you'll see a indication of how many minutes will take for that parking to be available again.
If you see such message for every parking, and it doesn't change until the AI goes away, it's because you are using UT2, and UT2 doesn't use standard traffic with normal schedules, but creates traffic on the fly and injects into the sim using Simconnect so, as far as FSX is concerned, that traffic is not scheduled. Which means, we can't read its ETD, so we can't generate a more meaningful message.