I tried to do something for having gemtext available for Mousepad (Xfce default editor) and I learned that is a GTK/Gnome piece.
Unfortunately this task is not trivial as I hoped, and the documentation is old and obscure (at least for me).
Syntax has been already laid off but it is not working with "gtksourceview3", I don't think I'll be able to address anything, not in the short term, but if you want jump in here is the link:
https://forum.xfce.org/viewtopic.php?id=15957
2 years ago
https://forum.xfce.org/viewtopic.php?id=15957