What does Clip Gain mean?
Clip Gain—also known as Region Gain—is a feature that allows the level of the audio on a track to be changed in addition to the level set with the Channel Fader.
Pro Tools® 10 ‒ Clip Gain Overview ‒ Avid - YouTube
No Pain, No Gain
One feature of Pro Tools that I've recently become envious of is Clip Gain: the ability to adjust the level of a region within a track without using the volume fader or automation. You can imagine how excited I was to learn that Logic has its own version of Clip Gain.
When you convert clip gain settings to volume automation, the clip gain settings are cut from the Clip Gain Line and pasted to the volume automation graph at the same timeline locations as in the clip.
Pro Tools Basics: Clip Gain Lines - YouTube
Clips do not store the audio or MIDI information, but do refer to the original audio or MIDI files of the session. Clips store information such as position, length and fades. Pro Tools presents a Clips List that can be access to select, clear, delete and manipulate Clips from your session.
Clip Gain is a new feature within Pro Tools version 10 which allows you to make level changes to an audio clip, and rather than just listening to the differences, you will also see a difference in the waveform meaning you can very quickly bring different dynamics in line with one another.
Select multiple clips, hold down CTRL+SHFT and use the UP/DOWN Arrow keys or scroll wheel to adjust gain. You can do this with the clip gain line turned on or off.
In terms of mixing, First lacks a seriously useful tool called Clip Gain. Clip Gain in the full DAW lets you turn individual audio clips up or down in volume. This is indispensable when mixing in a variety of ways.
How To : Clip Gain Workflow In Logic X (Quick video) - YouTube
Where is clip gain in logic?
FYI Logic has such a feature, region gain, over on the left of the main window in the inspector. Select a region (that is what I presume you are calling a "clip") then use the gain feature in the inspector to change the volume of just that region.
Select the notes you want to adjust in the Audio Track Editor. Drag the Gain slider in the Audio Track Editor inspector horizontally to adjust the gain.

Gain Staging is the act of managing the levels of your track. Automation can then give you the control to increase or decrease the volume so that it sounds equal and at a similar level throughout. By implementing gain staging and automation in your mix, you can immediately make your track sound more professional.
Reaper Tutorial Reaper Tips and tricks Pt1 Clip Gain, Item fx and Wet ...
Cubase Basics - Clip Gain & Gain Staging - YouTube
Ctrl+Shift+Up/Down Arrow: Adjust Clip Gain
This allows sound editors to essentially premix their work with very little effort. With a clip or group of clips selected, use Ctrl+Shift+Up Arrow or Down Arrow to adjust clip gain.
To automate volume in Pro Tools, first change the Track View selector from the default “Waveform,” to “Volume.” The black line that now appears across the track represents the volume, or in actuality, where the fader sits at any given time.
Adjust volume for all tracks in ProTools - YouTube
Description: The clip effect will hide or show an element by clipping the element vertically or horizontally.
Using the Clip List in Pro Tools - YouTube
What is a MIDI clip?
A MIDI clip in Live contains notes and controller data for playing a MIDI instrument.
Trim mode modifies the behaviour of Touch, Latch and Write modes, such that instead of writing over the existing Volume and Send Level automation lanes, a new Trim automation lane is created, which combines with the existing automation.
Then hold Alt-Shift while dragging one of the faders in the bunch(I didn't say "group" to avoid confusion). But you can also create a quick group, make your adjustment, and then disable or delete the group. Dragging faders, one or many holding the ALT key will cause the selected faders to go to 0.
You just hold option+shift and adjust pretty much anything(Faders, Mute, Volume etc..) on the selected tracks.
Pro Tools 11 - #12 - Link Track and Edit Selection - YouTube