Primary use: blog/homepage links in {{InfoBox user}}. LocalSettings.php has to be modified so it contains: array_push( $wgUrlProtocols, "gemini://" ); Documentation: https://www.mediawiki.org/wiki/Manual:$wgUrlProtocols
The gemini protocol has been enabled for external links as requested. I have left a test link on your discussion page. https://wiki.gentoo.org/index.php?title=User_talk:CyberTailor&oldid=1078538