It may be you need to enter the giffgaff APN settings. use this link.https://community.giffgaff.com/t5/Settings-for-your-device/tkb-p/phone_settings
Check in your phone settings to see if your APN is set to giffgaff or 02.
If it's 02 you need to CREATE a new APN inputting the giffgaff settings and remember to save them do not try to overwrite the 02 settings you must CREATE a new one and when created, SAVE it.
Once it's done giffgaff will show up as a selection in your APN list, just tap the dot to the side it will turn green and your done. Reboot the phone and BINGO. it works.