Gtk.FontSelection.prototype.set_font_name

function set_font_name(fontname:String):Boolean {
    // Gjs wrapper for gtk_font_selection_set_font_name()
}

Sets the currently-selected font.

Note that the fontsel needs to know the screen in which it will appear for this to work; this can be guaranteed by simply making sure that the fontsel is inserted in a toplevel window before you call this function.