If you are working with SharePoint Online and you go to Site Settings you might not see Web Designer Galleries displayed like so:
The reason that it no longer appears is because many of the Gallery options require scripting and this comes with security issues. So, Microsoft has elected to disable this by default.
To re-enable this scripting option so you can see the Web Designer Galleries, do the following:
Login to the Office 365 portal as an administrator and select the Admin icon from the App launcher.
In the bottom left of the Admin portal, select SharePoint under Admin.
This will take you to the Sharepoint admin center as shown above.
Select Settings at the bottom of the list on the left.
Select the option to Allow and save the changes.
It may take up to 24 hours for these changes to flow through to SharePoint Online and until you can see the Web Designer Galleries options in Site Settings.
For more information on enabling these scripting options in Settings from Microsoft visit:
CIAOPS 