Hi,
I had similiar problem when I installed 1.9 Update only -> there was following situation: I changed pushback direction from "Left" to "Both". After this, main GSX parking menu appeared, but when I choose "5 - Prepare for push-back and departure" nothing happens (GSX menu disappeared).
After reading a few posts, I installed GSX from full installer... but when I want to start pushback (choose option 5. in main GSX menu), the GSX window disappeared too. I had to Restart Couatl and start pushback again - then it works.
After installing full 1.9 version I made 2 or 3 flights and this happened one time. I'll observe it...
With regards,
Lucas