Why does the young man's voice catch when Gortsby lends him the sovereign?
It appears that the young man's emotional reaction may have been part of his act. The text tells us that, after the brief show of emotion, he quickly blurts out a word or two of thanks before he rushes off in the direction of Knightsbridge.
For his part, Gortsby is none the wiser, as he ponders the young man and his predicament:
"Poor boy, he as nearly as possible broke down," said Gortsby to himself. "I don't wonder either; the relief from his quandary must have been acute. It's a lesson to me not to be too clever in judging by circumstances."
Gortsby had originally doubted his youthful acquaintance's story when he heard it. The young man had claimed to have forgotten the name of his hotel after having left to purchase some soap. Since he cannot produce the soap he had supposedly bought, Gortsby finds himself a little skeptical about the young man's story. However, when he later spies the supposed bar of soap on the ground by the side of the bench, Gortsby rushes to the young man to lend him a sovereign.
In Gortsby's mind, he is lending the sovereign to a possibly wealthy young man. Who knows how the connection will eventually benefit him? So, Gortsby thinks to ingratiate himself to the young man, but it turns out badly in the end. Essentially, Gortsby finds that he has been at the mercy of a wily conman, certainly not a very pleasant realization on his part.
As Gortsby retraced his steps past the seat where the little drama had taken place he saw an elderly gentleman poking and peering beneath it and on all sides of it, and recognised his earlier fellow occupant.
"Have you lost anything, sir?" he asked.
"Yes, sir, a cake of soap."