Let me get this straight ... you have ALL the slots filled on a 6 channel RX and
you think it's not "being fully utilized" ???
You may want to rethink that.

If "something" is plugged into ALL 6 channel slots on the RX then the RX/Channels are being
fully utilized and there is NO place to plug the retracts ... period!

Your options are (as previously posted) get a 7+ channel system (TX & RX)

or ... go down the list of currently used channels for your heli and see if there is any channel you can do without and still fly ...

1-Aileron - not likely - no help
2-Elevator - not likely - no help
3-Throttle - not likely - no help
4-Rudder (T/R via Gyro) - not likely - no help
5- Gyro Mode/Gain - maybe - depends on the requirements of your gyro ... which you never stated what model gyro you have.
6-Pitch - not likely - no help

IF (that's a big if) ... if your gyro can be operated without the Mode/Gain lead plugged into channel 5 (and still have a fully flyable/controllable heli) then maybe you can disconnect
the gyro from channel 5 and plug your retract(s) servo into channel 5. Just remember to disable the gyro function so that the TX treats channel 5 as GEAR and not GYRO.

BTW ... just because you have a switch that does nothing doesn't necessarily mean that the channel does nothing. You can set the gyro mode/gain (channel 5) to change as your flight mode changes via
the flight mode switch leaving the CH5/GEAR switch unused. Does this mean ch5 is unused and available ... absolutely not ... it just means the SWITCH is unused.
Besides " ... ch6/RUD/D/R ... ". First ... RUDder D/R (dual rate) should be on Channel 4 ... not 6. Channel 6 is collective pitch which should be controlled via your ATVs and flight mode/Pitch curves.
Is this the switch you are referring to??

Looks to me like d/r control for Ail (ch1), Elev (ch2), and Rud (ch4). How did you get " ... ch6/RUD/D/R ... " from that???