First, be sure you have the very latest version. If you just updated the code with Live Update, without updating also the vehicles (as indicated on the release notes and on the forum), it's likely you'll have problems.
If you are not sure, download and install the latest GSX Full installer.
If that doesn't fix it, please, indicate a precise, exact reproduction procedure:
- The airplane used. If it's not a default one, clearly indicate the model and if customized it or not.
- The airport used. If it's not a default one, clearly indicate the scenery, and if you customized it or not.
- The gate used.
- The GSX sequence of operations you chose.
POSSIBLY, try to indicate a case when this happens with a DEFAULT airplane, with a DEFAULT or an FSDT scenery.