Gortsby sounds like a street-smart urbanite who has heard all kinds of sob stories, but he seems too generous and too gullible to be a confidence trickster. Why should he give a sovereign to a total stranger whether the young man's story was true or not? A sovereign was a gold coin worth one pound. A pound in those days was worth about five American dollars. Many of us might be willing to lend a stranger a dollar, but how many of us would hand over five dollars even today? And five dollars in those days was worth at least twenty dollars in current buying power. Saki (H. H. Munro) died in 1916.
I don't think there is anything in the text that would allow us to infer that Gortsby was a trickster once. Certainly he does think himself to be a rather shrewd judge of character, but that doesn't necessarily mean that he himself was a trickster too, rather that he is just an intelligent man who is not gullible and will not be taken in easily.
I think it certainly is possible that he was a trickster at one time. The main reason for me to say this is that he is very skeptical about the young man. The young man's story would probably have taken in many people since he was well-dressed and all. Gortsby doesn't believe him until the "proof" shows up in the form of the bar of soap. So you might infer that he had been something of a trickster himself since he seems to be on his guard for con men.
Very little is known about the main character of Saki's "Dusk." When he arrives at the park near Hyde Park Corner. Saki's writes,
He was in the mood to count himself among the defeated. Money troubles did not press on him; had he so wished he could have strolled into the thoroughfares of light and noise, and taken his place among the jostling ranks of those who enjoyed prosperity or struggled for it. He had failed in a more subtle ambition, and for the moment he was heartsore and disillusionised, and not disinclined to take a certain cynical pleasure in observing and labelling his fellow wanderers as they went their ways in the dark stretches between the lamp-lights.
Now, because Saki states that Gortsby had no money troubles, the reader may assume that he is not, in fact, a trickster. Instead, as the narrator states, he has failed in a more "subtle ambition," a phrase that suggests, along with his being "cynical," that Gortsby has become disillusioned regarding a personal relationship. Perhaps, he has had the "ambition" of marrying a young lady of some social prestige and he has learned that she has chosen another man of a higher social status than he. If this has occurred, it seems reasonable that Gortsby has become distrusting and cynical about people, for his love interest has probably been deceptive and misleading in the past. This past experience would also encourage Gortsby to be mistrusting of the young man who sits beside him with a tale of woe. However, if Gortsby is cynical because he has been recently been deceived in love, how bitter the irony of his realization at the end of the story.