In the system settings under Channel Assign, scroll down to Aux2 and make sure it says Aux2. Then scroll all the way to the right to Next and assign Aux2 to the switch you want to use for tail gyro gain.
Once done, you will go into the tail gyro menu in the Tx and if it says Inhibit, select it and click on it then move the switch you want to use for the tail gain and it will automatically select it. In my case, it is Switch D. Once you do that, you will see more options come up including the "Chan." or Channel. In there, you will set it from Gear to Aux2.
Most likely you will ONLY have to do the second part of that. The first part should be already done for you with stock programming.