I get this questions many times a week, so now it is officially a Tip of the Week: "Why are my Yaw channel settings grayed out in the HoverflyGIMBAL Setup Client?"
There are two reasons. Because we assume that the yaw servo will be a 360 degree continuous servo, some of these settings just don't apply. The rest are largely governed by the Mode Switch and setting them manually can result in issues that could lead to damage to your aircraft. So for safety's sake, only the settings that need to be available are.
If you should want to make adjustments to the Yaw servo's movement characteristics, Mode-specific changes can be made by following this path:
"Configuration & Calibration" > "Receiver Configuration" > "Channel 3"
Here you will be able to change the maximum deflection and speed of the Yaw servo by clicking the "Set Range" button for Current Angle (for Mode 1) and Offset Speed (for Mode 2).