In his lecture on the Victorian Age, Dale Ahlquist states that it was a period of the victory of the rich over the poor, as well as a period characterized by Utilitarianism. This philosophy was responsible for "aetheist industrialism." In addition, Utilitarianism, Ahlquist states, hinted at not "allowing the unfit to survive."
Dickens, however, sympathized with the desolate and the oppressed. In the grey atmosphere of almost gothic tones with Satis House and the marshes, Dickens injects the warmth and glow of the love at the forge and the friendship that develops between Pip and Herbert. That good can overcome evil is thematically developed with the emergence of the goodly characters Joe and Magwitch, who prove themselves nobler than any of the rich. (While the spiritually devastated Miss Havisham repents of her evil intentions against Pip, begging for forgiveness, she does perish.) Finally, in the Third Stage, Pip learns the true values of life: love and family and friends.
Thus, the strong atmosphere of "Great Expectations" is created in the tension between the grey Utilitarism philosophy/Darwinism of the Victorian Age in conflict with Dickens's belief in the basic goodness and worth of the "uncommonly good"--as Joe once calls little Pip--"common" people--as Estella derides Pip.
If you are referring to the opening of the book, Dickens' sets the novel's bleak mood at the very beginning. He describes the isolated marsh near which Pip lives, the grey weather, the cemetery with Pip's parents' and little deceased siblings' graves, and the ominous convict.
Throughout the novel, Dickens follows this pattern by using weather, time of day or night, and morbid locations (Miss Havisham's house, the cemetery, and later debtor's prison) to reflect the harsh times in Pip's life. While the novel does end on a more positive note, overall, its atmosphere reflects struggle from birth to live in an often unforgiving world.