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The number of knives needed by the restaurant is 587 - 387 = <<587-387=200>>200 knives.
Each set on sale contains 20 knives, so the number of sets Gabriel should buy is s = 200/20 = <<200/20=10>>10 sets.
Therefore, the inequality representing s is s ≥ 10. Answer: \boxed{s \geq 10}.