User Preferences don't offer Gadgets
Unfortunately, it's a local wiki like I wrote. Special:Version shows Gadgets in the section Others.
I could search by myself if I could read the current installation and configuration of de-Wikibooks. But I don't know how to check LocalSettings.php and all the files in the includes folder like SpecialPage, SpecialPageFactory, and so on if there are relevant differences between WB and my own configuration.