To track online purchases made using the ‘Continue as guest’ option:
Navigate to Reports (center level) > Marketing > select Gift Cards from the filter > select Details.
Select the card type (Amount, Service, or All cards) and date from the Filter Criteria and click Refresh.
In the Guest column, gift cards purchased online without the guest signing-in, appear as "Online Guest".
You can export the report to view the details of all such purchases made on your webstore.
To learn more about the report, read: Gift Card Report: Details.