Let me answer that for you.

Gold customers will never come again until very late in the game.

They appear only when the non-gold non-closer customers left are less than the amoount required for the day. When that happens, a random Gold Customer will pop up (and surprise the toppings out of me, in my case) and take his usual order.

When they would normally progress in rank, you instead get extra points and tips, and the remain gold customers.

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