IMO, the association is and should be closely related to the position in the flow. The 'Gain' knob is at the input, where MIDI lives in an instrument track. ProChannel is post instrument, so it's audio.
It's really obvious what the function is. If the range is integral values from -127 thru +127, it's a velocity offset control, or "MIDI Gain". OTOH, if the range is -18.0 thru +18.0, it's an "Audio Gain" control.
And aren't the "Gain" controls on audio tracks really the old "Input Trims"? There is no audio input in an instrument track, soooooo ...
Just sayin' I can see The Pastry Shoppe view on this. Could there be a better name? Sure, but is a poorly named control worth the added health care costs down the road?
Then again, I don't use simple instrument tracks, so carry on!