In "Dusk" by Saki, why do you think that the young man's look of disarming frankness puts Gortsby on his guard?

It is evident from the description of Gortsby's observations and reflections that he makes a habit of sitting on a bench in this general area around dusk, probably shortly after he gets off work in some office, and watches the passing crowd. This would inevitably mean that he would be approached by strangers with hard-luck stories. Gortsby considers himself sophisticated. No doubt he has given people money in the past and then had second thoughts about it. He has obviously developed a cynical attitude about humanity in general as a means of self-defense. When a man sits down beside him on his bench--especially when there are probably vacant benches all around at that late hour--he is immediately suspicious. He is familiar with the stranger's opening ploy.

As if to emphasise the fact that the world went badly with him the new-corner unburdened himself of an angry and very audible expletive as he flung himself into the seat.

Gortsby understands that this "audible expletive" is intended to get him to ask what is the matter. He does as expected because he is amused. Whatever the young man is going to ask him for, the answer will be no. Gortsby is doubly suspicious and defensive with the young man's response to his question:

The young man turned to him with a look of disarming frankness which put him instantly on his guard.

Gortsby has seen that well-rehearsed "look of disarming frankness" before. It is always the prelude to a hard-luck story. It is an act that naturally seems to go with hard-luck stories. It might be effective with some people who have never heard such stories or seen such looks--but part of the price that Gortsby has to pay for sitting on a bench in this park at dusk is being a target for panhandlers and con-men of all denominations, from those who are only asking for a penny to those who are after as much as a pound. Gortsby has hardened his heart as a result of experience. 

Since Gortsby is suspicious of this young stranger from the beginning, he is listening critically and detects the one flaw in the otherwise clever story. 

"Of course," said Gortsby slowly, "the weak point of your story is that you can't produce the soap."

This con-man seems like a novice. He is stuck with claiming that he not only lost his hotel but that he lost the cake of soap he went out to buy with only a shilling in his pocket.  The con-man is angered--not because he was exposed, but because he has invested a certain amount of valuable time unproductively. It will be dark soon, and he will have a very short time left in which to find another "mark" to listen to his story. The fact that it is dusk means that people will be getting up and going home soon. 

The con-man's story was fashioned to give Gortsby the impression that he was being given a golden opportunity to make the acquaintance of a young country gentleman who might offer him opportunities to rise in the world. The stranger makes it clear that he is all alone in London. No doubt he would be happy to know a long-time resident his own age who could show him around town. The con-man's story contains several "hooks," one of which is:

There's a nice predicament for a fellow who hasn't any friends or connections in London! 

Gortsby, the cynic, is hooked by the prospect of gaining something for himself by making what is only a short-term loan of a sovereign. He can hardly doubt that this young aristocrat, who talks as if he has been to Eton and Oxford, would fail to repay the loan. But Gortsby will find out that he has been tricked again. And he may become even more cynical and hard-hearted as a result.