Can I give a customer a gift card automatically when they reach a milestone?

Yes. On Marketing, Gift cards, use the panel called Cards people earn. Pick what they have to do, how many, and what the card is worth, then save the rule. A hundred days of ordering earns a fifty card, and it arrives on its own with its code on WhatsApp. After you save the rule you do nothing. The docs tab Gift cards, store credit and house accounts has the full guide.