Configuration.
The general GeSHi Filter settings can be found by navigating to: Configuration > Content authoring > Geshifilter OR admin/config/content/formats/geshifilter.
If your library is detected, it should show something like this, GESHI LIBRARY VERSION 1.0.8.12 DETECTED.
If you use ckeditor that is default now in Drupal 8 i recommend that you change two settings. In Generic syntax highlighting tags add the "pre" tag, as
you can write inside a pre block without ckeditor changing every new line to <br> which will show in the output. Enable Decode entities because ckeditor will encode some chars and without this setting they will show in the encoded format in the page.
Now go to admin/config/content/formats (Configuration -> Content authoring -> Text formats and Editors). We have to enable the geshifilter in one or more text formats. So choose one text format and click on configure. Now just enable geshifilter and save.