But the BeastX does use the value of channel 5 to set the tail gain. So, you must do whatever is necessary to make that channel do what you want. If you want different gain settings per flight mode, the answer gets trickier. If you just want to control the gain with the gear switch or the AUX2 switch it is easier.
Easy first: The DeviceSEL menu controls which switch controls channel 5 and channel 7. Whichever you want, do the assignment there. Then, in TRVL ADJ. menu, set the end points to the gain you want. If you use the AUX 2 switch, don't leave the center position at zero, you don't want to have zero gain if you accidently choose the center position.
To select different gains depending on the flight mode switch it is a bit weirder. The 9303 uses the governor menu to control channel 5 and the gyro menu to control channel 7 and you cannot change this. So 2 ways to work around this limitation: 1) Use the gyro menu and do a mix from AUX2 to Gear in order to get the change on channel 5. OR 2) just use the governor menu to control the gain. Either will be confusing but will work.
In implementing all these choices, the monitor screen is your friend. If you don't see the value you want on channel 5 (GEAR) then the BeastX won't.