Gwendolyn is introduced to the reader as the object of a male gaze - Deronda's gaze - whose introspection sets the immediate tone and perception of Gwendolyn for the reader. And he also immediately associates evil with her beauty. She will remain the object of male gaze throughout the novel, as various men examine and interpret her looks, her words, and her imagined feelings.
When she reappears in the evening at the casino, this association with evil is further reinforced by "...she has got herself up as a sort of serpent now -- all green and silver, and winds her neck about a little more than usual" -- "It is a sort of Lamia beauty she has" -- again the object of male gaze. An interesting choice Lamia - who was cursed with eating her own children and permanent insomnia -- a punishment from Hera after having an affair with her husband Zeus.
And the anti-Semite theme is introduced early as well, with her unpleasant interaction at Mr. Wiener's pawnshop - "...these Jew dealers were so unscrupulous in taking advantage of Christians unfortunate at play!"