When you completed the purchase you would have also been sent the gift code via email to your PayPal-registered email address (sometimes, depending on your email provider though, these can end-up in your Spam or Junk folder).
If you still can't find it, get in touch with PayPal Digital Gifts customer support - they'll be able to track down your order and re-send the gift code to you:
https://www.paypal-gifts.com/us/contact-us/
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