GSX won't automatically switch gate if an AI took it while you were taxing, but you can do it yourself, but opening the menu before entering the gate, choose the option to Change the gate choose the same one. It will alert your gate is occupied, but it will ask if you want to use it anyway, and will kill the AI should you decide to do so.
Although, in real life you would probably have to choose another gate, which of course is always possible.