'Sounds like you have a garage in which you can/do park cars "two deep"?
If that indeed is the case, may I suggest you hang a rubber ball by a string from the ceiling ahead of the GT so that when the car you're backing in MOVES the ball/string you know to "whoa"? :wink :cheers
Some type of removable "parking curb" on the floor...a sheet of plywood mounted vertically in front of the GT...or a dozen other things would work, too. But, the suspended ball is pbly the simplest/easiest of any.