His mouth opened a little, and he looked at Gatsby, and then back at Daisy as if he had just recognized her as some one he knew a long time ago.
Tom is so astonished that his wife could ever possibly love another man that his jaw drops in an almost cliché moment. A powerful metaphor is used in the second half of the phrase to further emphasise his shock. He is looking at her with fresh eyes and has seen a little more of her personality than he saw before then.
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