As you're no doubt aware already JR transmitters, specifically with Futaba gyro's split the ATV on the channel controlling gain into two halves. In most cases it's as follows;
0~50% = 0 ~ 100% heading hold mode
51~100%= 0 ~ 100% rate mode
The above can also be the other way around, but you will soon work it out via the status LED on the gyro of the way the tail servo behaves when the model is moved.
This sometimes gives people problems as until you've sussed this then setting the gain level can be difficult.
The GY611 is easy to setup as it has the display to check is changing as you make changes to the ATV values.
With this I would set the gains to 38 in both AVCS and Rate mode, this means looking for 38A gain value in AVCS and 38N in rate mode shown on the gyro's amplifier box.
If your struggling with the gain menu in the DSX12 to setup your gyros, then just revert it back to a normal channel and use the ATV page to set the gain levels. This normally makes things much easier I find.
To use rate mode then the usual setup procedure applies once you've figured out how the gain channel setting is working and this is to trim the tail in rate mode, adjusting the tail pushrod and then flick into heading hold mode.
Hope this helps in some way?
Team Robbe SchluterUK