It's not possible to help you with a so sparse description of the problem. Please indicate all the following:
- the airport used
- the gate used
- if you customized the airport or downloaded a pre-made GSX customization. If yes, please indicate which one
- if you customized the airplane. Both FS Labs and PMDG are fully supported by GSX, so they don't require any customization.
- the precise, exact, sequence of operations you did in GSX
- if the problem happened on departure, or on landing only, or both.