The double negative in the words "not disinclined" means essentially the same as "inclined." The word "inclined" itself is a modification of the notion that Gortsby was enjoying other people's suffering because he was unhappy himself, for what reason the author does not state. Being young, it seems likely that Gortsby has been disappointed in love; but being an alter ego of the author Saki, it also seems likely that Gortsby might have had a novel or book of poems rejected.
It was some thirity minutes past six in an early March evening, and dusk had fallen heavily over the scene, dusk mitigated by some faint moonlight and many street lamps.
Saki has to specify that there are many gas lamps because he wants darkness to set the mood, as well as for other purposes, and yet he wants his viewpoint character to be able to see people's faces. Gortsby could hardly form so many opinions about the pedestrians he labels as "defeated" if he couldn't see them. Therefore, Saki specifies that
. . . they went their ways in the dark stretches between the lamp-lights.
They are only shadows as they pass between the street lamps, but then when they enter each circular zone of light their faces are briefly illuminated in a ghostly manner. The author also specifies that there is some faint moonlight--not that this would add much to the illumination of his somber scene, but in order to establish that it was not a bad evening to be out walking or sitting on a park bench, even in England in early March, which is still wintertime with the trees all barren of leaves, and as late as six-thirty.
Saki's main purpose in establishing that Gortsby was "not disinclined" to enjoy observing so much misery is to prepare for the change of heart his hero will experience when he finds the cake of soap under the bench and naturally assumes it belongs to the young man who lost his hotel. The point of Saki's story is that it is a mistake to get sentimental about people in distress. This thesis has political connotations.
Saki, a member of the English ruling class, has been described as a Tory and a reactionary. A "reactionary" has to be "reacting" againist something. The reactionary ideology implicit in "Dusk" is against the socialist and communist movements which were growing stronger in the early twentieth century and would shortly lead to really world-shaking reactions in Spain, Italy, and Germany.
Gortsby goes from being unsympathetic to the "defeated" to being sympathetic and then to being antipathetic after he is cheated out of a sovereign by a grifter. The story is about one young man's learning experience. (Whenever a fictional hero is described as "young," it is a clue that the author will use his experience to convey a message.) And Gortsby's lesson is that it is foolish to get sentimental about other people because they won't be sentimental about you. The young grifter is not a sympathetic figure. Would he ever lend a sovereign to anyone else? Life is a battle of all against all. The best philosophy is to look out for Number One. The poor you have always with you, as a prophet once said. The Old Testament puts it even more coldly:
The horseleach hath two daughters, crying, Give, give.
Saki uses dusk to symbolize his thesis that it is impossible to distinguish between people who need help and the majority who don't deserve help--that charity and generosity in this dog-eat-dog world are for suckers.