Try to enable logging in the Troubleshooting section of the GSX settings. Then try again (pressing the hotkey at a gate with engines off and brakes on), exit from FSX, and post your Couatl.LOG here, which is found under the %APPDATA%\Virtuali folder.
Don't quote or post the file inside a post, but ZIP it, and attach it to a message, using the attach function of the forum, under the "additional options..." of a message.