There are two way to define gadgets depending on the $wgGadgetsRepoClass. If It is 'MediaWikiGadgetsDefinitionRepo'(the default value), the list of available gadgets is defined on MediaWiki:Gadgets-definition. In an alternative way, Gadget definitions defined on pages in the Gadget definition namespace when $wgGadgetsRepoClass is set to 'GadgetDefinitionNamespaceRepo'. CAUTION: Migration of the existing gadget definitions is not yet supported, So you will lose previously defined gadgets after changing this option.
Once created with at least one valid gadget, gadgets defined there show up in the "Gadgets" section of Special:Preferences, so users can pick the gadgets they would like to use. An overview of the gadgets defined by MediaWiki:Gadgets-definition is also shown on Special:Gadgets, along with links to the respective system messages, for easy editing. Statistics for non-default gadget usage can be seen at Special:GadgetUsage.