On the microphone and speaker gain controls balloons display, use/show the same units as the meter. i.e. dB/% (linear).
For speaker volume this would be possible, for mic volume it would not be: The mic volume currently is tied to the OS mic volume, which has a dynamic range of just 36dB iirc. So technically the mic volume slider is using a different scale to the meter it sits on.
Unity gain would be the 0dB reference and it would go +/-ndB. Regardless of what the range is.
Seems like it ought to be possible. Percent and dB are both representations of ratio.
EDIT to add some info.
Windows 8.1 and 11 (probably 7 & 10 version too) allow microphone, line and speaker gain to be set by dB or percentage. The following images are of microphone on Windows 11. It is same/similar for other devices and Windows 8.1.
