glossaries package FAQ
You need to use a glossary style that supports this. For the predefined styles provided with the base glossaries package, these are the styles whose name ends with "group". For example,
treegroup. The glossaries-extra package provides an additional style
bookindex in the accompanying glossary-bookindex package that also supports letter group titles. See the gallery of all predefined styles for examples.
If you are using glossaries-extra with bib2gls, you additionally need to remember to call bib2gls with the
Alternative link: https://www.dickimaw-books.com/faq.php?itemlabel=groupstyles