Hi Shannon_hong,
As media gallery manager (depending on KAF role mapping, by default a teacher/instructor) you should have a "remove" button in the gallery view (before clicking the "add media" button).
You need to click the + sign on each item - you will see extra information and the button to remove it.
Note that this does not delete the item from your Kaltura account. It only removes it from the current gallery.
The owner of the media, depending on permissions, can re-publish it into the gallery.