Returns (at runtime) the absolute path of the input GKK file.

def gkk_filepath(self):
        """Returns (at runtime) the absolute path of the input GKK file."""
        if self.gkk_node is None: return "GKK_FILE_DOES_NOT_EXIST"
        if isinstance(self.gkk_node, FileNode): return self.gkk_node.filepath

        path = self.gkk_node.outdir.has_abiext("GKK")
        return path if path else "GKK_FILE_DOES_NOT_EXIST"