I don't believe Pinhead has a soul. Because at the end of Hellbound you see the original explorer who became Pinhead rise to confront him. It's still debatable though.
But anyway if the stare destroyed the explorer's soul Pinhead may still remain...he may even become stronger because all traces of humanity have been erased, he'll become a true demon then.
Pinhead would still have a mad battle on his hands. Although he can manipulate reality to create weapons out of thin air to hold GR, GR would just brake his bonds and unleash punishing blows. Pinhead always liked to send projectiles and hook chains at his opponents....they won't do crap to Blaze.