I recently switched from gedit to mousepad as part of getting fed up with Gnome and switching to Xfce.
Though mousepad modestly describes itself as “a simple text editor”, a pleasant surprise is it probably supports syntax highlighting for more programing languages and data types than anything I’ve encountered outside of Emacs.
Besides Prolog, mousepad also does Logtalk. It supports so many different syntaxes that Document > Filetype takes you to five options (Markup, Other, Scientific, Script, Source) and Source is the one you’ll find Prolog in.
These days Atom seems to be the favoured open source text editor, and besides Prolog text highlighting it does linting, but I personally hate IDEs that obscure what I’m typing behind pop-up error boxes while I’m mid sentence.