use the system monospace font in gtk textview

[available since 3.16]

set_monospace()

GTK3+ Doc https://developer.gnome.org/gtk3/stable/GtkTextView.html#gtk-text-view-set-monospace

gtk_text_view_set_monospace ()

void

gtk_text_view_set_monospace (GtkTextView *text_view, gboolean monospace);

Sets the “monospace” property, which indicates that the text view should use monospace fonts.

Parameters

text_view a GtkTextView

monospace TRUE to request monospace styling

You can just use "monospace 18" as your font and it will use the system monospaced font.