To export the GreenUI 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:

* GreenUI[ResourceLoader|default|hidden|targets=desktop,mobile|skins=minerva,timeless,onyx]|GreenUI.css

You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.