Finished a flight today from KFLL to KATL in my PMDG 737. Everything worked great at KFLL. When I arrived in KATL I noticed all the gates said "Jetway Unavailable" in GSX. I opened up the editor and noticed jetways had been turned off for all gates at KATL. Decided I'll worry about this later as I wanted to finish my turn around to KFLL.
This clearly indicates the problem doesn't have anything to do with GSX, it was caused by the very well-known problem of Simconnect stop working when the number of objects in the sim (currently documented at 1000) has been reached. With a large airport like KATL, it's possible that AI and default ground vehicles might be enough to reach it.
It Simconnect stops working, the call GSX makes to the Navdata API to get a list of jetways (and other airport data) will fail, so GSX will assume there are no jetways at the airport.
Note that, it's not that Simconnect completely stops, but the Navdata part is, and fact it's a Simconnect problem is further confirmed by the fact is not solved by restarting Couatl, because the problem is happening in the simulator itself.
Now, if you said more precisely what do you mean with "KATL" (lacking any details, we can only assume it's the default scenery) but, just to be sure, if it's a 3rd party airport or a handcrafted airport, have you DISABLED GSX replacement Jetways there, something you are always supposed to do when adding a new airport after installing GSX ? (when you install GSX or update GSX, it will do it automatically, for the airports it can recognize automatically).
If you don't disable GSX replacement jetway files for non-default airports, you will introduce a scenery conflict that WILL cause every kind of issues with jetways.
Only way to make GSX work is to not use the PMDG 737.
I'm sorry but, there are no issues whatsoever using GSX with the PMDG 737, it's in fact the best supporter airplane.