The Gain on a gyro controlls how much the gyro makes the tail servo move - in other words the sensitivity of the gyro.

if the heli in 'normal' curcumstances rotates by itself - you have to turn the gain up

if the heli oscillates (moves contantly left and right) the gain its too high and you have to turn the gain down.

Your problem: it sounds like your heli is getting interfereance and you said it has to do with the throttle SO is your motor suppressed?
if it isnt it will be making RFI.
it your motor is suppresses i dont know what is wrong with your heli maybe a board is mashed up or something