You need to use a condition. Set the condition to turn on when the gear is in the up position. Turn on the condition. Go into the Model Menu and then the AFR section. Go to the nosegear steering channel. Set it to "Single". Adjust Rate A and Rate B to 0%. This will give you a flat, horizontal line. Now, when you raise the gear, the nosewheel will center and remain there no matter how you move the rudder. When you lower the gear, the original endpoint setting will take over.