Gtk.TextIter.prototype.toggles_tag

function toggles_tag(tag:Gtk.TextTag):Boolean {
    // Gjs wrapper for gtk_text_iter_toggles_tag()
}

This is equivalent to (gtk_text_iter_begins_tag () || gtk_text_iter_ends_tag ()), i.e. it tells you whether a range with tag applied to it begins <emphasis>or</emphasis> ends at iter.