To export the page_length 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:
* page_length [ResourceLoader|hidden|default|type=general] | page_length.js
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.