Most of the time when I request a deboarding / boarding all necessary equipment but the passenger bus and tractor of the baggage train does not appear anywhere around the airport, so the process suspends.
That doesn't happen here, and nobody else reported it before. Which airplane, airport and gate you used ?
But generally a single reset does not work and it needs to reset three or four times.
That, instead, it's really impossible because, a "Reset position" will restart the *whole* Couatl.exe program and, from the point of view of GSX (which runs entirely inside it) it's exactly the same as restarting FSX so, it's not really possible that you would see a difference between resetting once or more than once.
Unless, your system is very slow, and restarting the program takes a while so, you *think* you have reset 3-4 times, but in fact it was just one reset all along.
GSX does not sense the door situation (open or close) and assumes that all doors were closed at the start of the process and gives warnings to make me open a door which is already open and holds the process until I cycle the door
It surely does, and it doesn't do what you are describing. The status is correctly read in any case, and it surely works. IF your airplane uses custom doors commands, and is not in the list of the supported one, there's no status at all to be read, GSX won't read it at all.
Again, which airplane you used ? Which other addons you have ? Perhaps they are interfering with the doors in some ways ?