The problem doesn't have anything to do with GSX or supposedly "bugged" animation. Is simply caused by the airplane not being configured, or being configured wrong. There's internal support in GSX for the Justflight 757, but ONLY the "Freemium" version, which is named this way:
JF_B757-200_RR_FREE
So, every other variant of the airplane must be configured manually, using the airplane configuration editor page that comes with GSX.