To export the rollback-summary gadget, click on "Download" button, save the downloaded file, go to Special:Import on the destination wiki and upload it. Then add the following to MediaWiki:Gadgets-definition page:
* rollback-summary[ResourceLoader|type=general|rights=rollback|dependencies=ext.gadget.libOOUIDialog,mediawiki.api,mediawiki.Uri,ext.gadget.site-lib|default]|rollback-summary.js
You must have appropriate permissions on the destination wiki (including the right to edit system messages) and import from file uploads must be enabled.