I went to the Icons gallery, but could not find any means of searching within them. Am I missing something ?
Look in Icons gallery here:
And Object Gallery here (not "Object Dock Backgrounds" Gallery):
In the Object dock gallery dock backgrounds will be mixed in.
To search within either one of them, go to the gallery and then type a word or words in the search box at the top right of the page about 1/2" from your login name. Keep an eye on the little icon at the left of the search box - if it changes to "All" you'll be searching all of the galleries.
Here's what a search in Object Dock for "3D" yields: