Getting full gain without heading hold from the GYA350, or any other Futaba gyro is quite easy.

If the gain control channel is set to -100% you get 100% gain in normal mode. Set the control channel to +100% and you get 100% HH gain. Set the control channel to 0% (the center position) and you have zero gyro gain.

If you want 50% normal gain, set the gain control channel to -50%.

If your radio displays the channel output as 0% to 100% instead of -100% to +100%, then the gain would work like this:

Radio
Settingʰʰʰ Gain
===============
0% ʰʰʰ 100% normal
25% ʰʰʰ 50% normal
50% ʰʰʰ 0%
75% ʰʰʰ 50% HH
100%ʰʰʰ 100% HH