I can't blame any fighter (aside from Penn) for calling out GSP. Win or lose, a fight with GSP is like a fight with Conor because it's red panty night after that. There's more money in a GSP fight than any other fight at MW, WW or LW, including the Diaz vs McGregor fight.
But Penn can't call out a guy who embarrassed the shit out of him in his prime and has demonstrated a significant downward spiral since then.