gather-hide notifications have two secondary links, one to view the moderation criteria and one to contest the hiding of the collection. AFAIK these are the only secondary links that no icons have been chosen for yet.
I'd like to solve a more general case first: define a generic default action icon for those actions lacking one.
Using a caret pointing forward (as we do for "All notifications" in the panel footer) seems a good candidate.
The caret will be pointing right (
For the specific case of "gather-hide", we can leave the action placeholder icon (caret) for the moderation criteria and use the speech bubbles () for the "contest" action. I illustrated this below (trying also to shorten the notification copy a bit):