I think maybe there is a little misunderstanding of what’s being talked about here; i.e. - ‘Clip Gain’ vs ‘Volume Envelope’.
The former (little white dot at the center-top of any clip) globally adjusts the overall clip ‘gain’, up or down only - as @djw says, using the Info Line for exact values to match or otherwise, to other clips etc. The latter is the Blue Line on which you can add multiple data points, across the length (Envelope) of the clip.
The manual will be a better friend for the detail… 
(Please accept my apologies if I’ve read this wrong.!)
Cheers,
puma