The truth is that Gortsby relies too much on logic in ascertaining whether the young man is truthful or not, and not enough on his own judgement and intuition. Notice the way in which Gortsby quickly hones in on the one problematic point in the young man's story that would have proved it to be true. Consider what Gortsby concludes as he dwells on the young man's case:
"It was a pity," must Gortsby; "the going out to get one's own soap was the one convincing touch in the whole story, and yet it was just that little detail that brought him to grief. If he had had the brilliant forethought to provide himself with a cake of soap, wrapped and sealed with all the solicitude of the chemist's counter, he would have been a genius in his particular line. In his particular line genius certainly consists of an infinite capacity for taking precautions."
Even though Gortsby seems to intuitively recognise that the young man is a confidence trickster, his over-dependence on the power of logic means that when he finds the bar of soap by the bench his other powers of reasoning are overruled and he comes to the erroneous conclusion that the young man must have been speaking the truth. Thus Gortsby's mistake, to my mind, is the result of his reliance on logic to the exclusion of all other factors in determining the veracity or otherwise of the young man.
yes I agree because firstly he did not believe the youngman as he could not show the soap then he overruled his own decision in a haste and went to search the youngman .gorsby did not notice the youngmans hostility and gave him a sovereign as loan too.he was proved wrong again when he saw the old gentleman looking for his soap.