Gadget export
To export the Merge gadget, click on "Download" button, save the downloaded file, go to Special:Import on destination wiki and upload it. Then add the following to MediaWiki:Gadgets-definition page:
* Merge[ResourceLoader|dependencies=mediawiki.api,mediawiki.util,mediawiki.language,oojs-ui-windows,wikibase,wikibase.sites|rights=delete]|Merge.js|EmptyDetect.js
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.