No need to overhaul to GB if the aux pump is leaking.
However, you need to dismantle the GB from the engine in order to change the pump.
The cost should be about this amount:-
1. New pump - @RM1500
2. New ATF - @400 to 550
3. Labour - @ RM600
change GB mounting as well and also check the torque converter sleeve. If it is worn out, there would be also leak.