NOTE: This is NOT available to all users/in all apps
Click on the icon shown when editing text to add an App/Content Block placeholder.
Save the content and you will then see the "App / Content" placeholder in the content.
Hover over the App / Content placeholder and select the Content Block or App options.
NOTE: The Inner Content placeholder has light blue icons to distinguish from the Page Content icons which are dark blue.
The Gallery App has been added to the Rich Text Editor as Inner Content