The Gorn were not zeroing in on Spock's tricorder readings. They were using area bombardment in addition to very accurate, point disruptor fire (which of course was not aimed at Spock).
As an aside, I've always been disappointed that the Gorn were not used more in 'Trek. Their ship (sadly unseen) shrugged off both phasers and photon torpedoes, which neither Romulan, Klingon, nor Federation ships of the time could do. They seemed to have good tactical sense, both in setting their trap and in employing multiple simultaneous attack forms against the landing party. And, of course, they look cool (the nose hairs need to go, though).