Try using this link: https://giffgaff.com/orders/history/others
On that page, either go through the pages, or choose the option to 'Show 100 entries'. This should help you find the purchase. You might have to scroll for a while depending on how many purchases you have made since buying the phone.
Also make sure you bought it on this giffgaff account. Maybe you had forgotten to sign in when buying the phone so it is on another account?
Also, as @boblog mentioned, check your email invoice for the order to check which account it was bought on and see if you can view the order through there.
If you still have no luck, you can contact an agent at this link: https://support2.giffgaff.com/app/ask/Phones/Phone-repair/form/
Make sure to fully explain the situation to them.
When you get a response, you should receive a text and email alert telling you. You can view the response either from the bottom of your giffgaff dashboard, or by clicking this link: https://giffgaff.com/support/questions
If you still need help, feel free to reply to my message and mention me (@mrjeeves)