I have a few ideas to mind:
Have you tried to enter the APN settings again?
Try and connect to wifi and download the giffgaff apn app from Play Store.
You can then set up the APN by following the instructions and copying and pasting the info into the relevant fields.
Where was the phone purchased from?
Another issue could be that maybe the IMEI might be blocked due to the original owner reporting it lost or stolen?
Have you tried another sim in the phone to test if calls, texts and data work ok?
I hope this helps mate.