1. Set the track(s) Edit Filter to Clip Automation > Gain. If you hold Ctrl while changing one track's Edit Filter they'll all change.
2. Select the clips whose envelopes you want to edit. These can be in different tracks.
3. Shift+click on any of the envelopes with the Move or Edit tool, then drag.
Note this is for clips in tracks, not Take Lanes. Convert Take Lanes to tracks if you need to adjust Take Lane clip gains.